A teenager who boarded a commercial flight in Australia armed with a shotgun and ammunition was overpowered by passengers and crew, the police said on Friday.

The teenager, a 17-year-old boy whose identity was not released, had entered the aircraft with the firearm, the department said. Officers from the Victoria Police were called to Avalon Airport in southeastern Australia at about 2:20 p.m. local time on Thursday, it said. He was arrested and faced multiple charges.

A sheep shearer, Barry Clark, was among those onboard the flight who stepped in to detain the teenager. Mr. Clark said that he was headed to Sydney to watch a football match when he noticed a member of the crew asking the teenager, who was dressed in a high-visibility vest and was wearing a tool belt, for his credentials before entering the plane.

“He got agitated and before we know it there was a gun, a shotgun appeared,” Mr. Clark told ABC Radio Melbourne.

“I then proceeded to do what I had to do and just put him in a bit of a lock, got his hand and twisted it and put it up in his back, threw him to the floor and then put my knee in his back and held him in a position where he couldn’t get out,” Mr. Clark told Network 10, a television network, according to The Associated Press.

Police said that they found two bags and a vehicle belonging to the teenager and called in the bomb response unit to assist with the bags as a precaution. No one was injured, they said.

Jetstar Flight 610 was scheduled to fly to Sydney with about 150 passengers. The superintendent of the Victoria Police, Michael Reid, told reporters that the boy had climbed through a hole in the airport’s security fence and made his way to the stairs of the aircraft.

The teenager, from the regional city of Ballarat in Victoria, about 70 miles west of Melbourne, is in custody on eight charges, including unlawfully taking control of an aircraft; endangering the safety of an aircraft; possessing dangerous goods on an aircraft; a bomb hoax; and firearms possession.

On Friday, he appeared in the Children’s Court of Victoria and told the magistrate that he was not applying for bail, according to the Australian broadcaster ABC News. The news report cited charging documents stating that he planted a fake homemade explosive and threatened passengers and crew.

“I’ve got bombs in my bag,” he was quoted as saying.

Mr. Clark, the shearer, said he did not hesitate to get involved, saying that he had been taught as a boy to look after other people. “You don’t think, you act,” he said. “I was quietly confident I could handle him.”

He did not answer the radio interviewer’s question about what was easier, wrangling an attacker or a sheep. Instead, Mr. Clark spoke about the employee who told him seat 1-C was “the best seat in the house” when he made his booking.

Next time, Mr. Clark said, “I am going to ask him if I can go down to cargo.”

The Trump administration has asked researchers and organizations whose work is conducted overseas to disclose ties to those regarded as hostile, including “entities associated with communist, socialist or totalitarian parties,” according to a questionnaire obtained by The New York Times.

The online survey was sent this week to groups working abroad to research diseases like H.I.V., gather surveillance data and strengthen public health systems. Recipients received funding from Centers for Disease Control and Prevention, the United States Agency for International Development and other federal sources.

The questionnaire appears to be very similar to one sent earlier this week to partners of the United States Agency for International Development, which has been all but dismantled by the Trump administration. Both were titled “Foreign Assistance Review.”

Recipients were instructed to respond within 48 hours. Some grantees interviewed by The Times feared that impolitic or unsatisfactory answers could lead to cancellation of funding.

“Taxpayer dollars must not fund dependency, socialism, corrupt regimes that oppose free enterprise, or intervene in internal matters of another sovereign nation,” the questionnaire said.

“A truly prosperous America prioritizes domestic growth, innovation, and economic strength over foreign handouts,” it added.

A spokesman for the White House did not return a request for comment about the survey.

The form asks a series of questions that touch on collaborators, including whether grant recipients work with drug cartels or “groups that promote mass migration.”

It asks about the impact of programs, such as whether they have an effect on “promoting religious freedom and combating Christian persecution” in other nations.

It asks grant recipients if they or their organizations collaborate with or have received funding from Russia, Cuba, Iran or China — including the Confucius Institute, which works with China’s Ministry of Education to promote language and cultural exchanges with American students.

Some federal officials who oversee grants said they were particularly dismayed by a question asking grantees to disavow associations with communist entities. The United States assists with health work in a number of communist countries, including Vietnam.

“Some of my grantees are literally the public health government structures in a communist country,” said one federal official, who spoke anonymously for fear of retribution. “This is painful.”

Awards and contracts to aid groups have been under review during a 90-day pause on foreign aid, but in some instances they have been canceled and restored over the past few weeks. The Supreme Court on Wednesday ordered the federal government to resume the payments.

Various versions of the questionnaires were sent out starting Wednesday, and the deadlines appear to range from midnight on Friday to 5 p.m. Monday. But the process has been riddled with confusion.

Some grantees appear to have been given extensions, while others were denied. Some noted that the deadline seemed to shift as more copies of the questionnaire were received each day.

The recipients included scientists at large universities and research programs in the United States, as well as tiny organizations abroad. Some questionnaires were apparently sent in error; one was received by Maine’s health department.

A principal investigator with two C.D.C. grants for domestic research projects received the email Thursday evening, only to receive another Friday morning telling him to ignore the previous missive.

The answers are meant to be filled out by a single person in one sitting, with no way to save responses for a break. The form estimates the task to take 30 minutes, including the time required for gathering the necessary data and documents.

“The chaotic way in which these stop work orders, terminations, waivers, reinstated awards, surveys — all with rapid deadlines and minimal communication — have been sent is distracting and incredibly disruptive,” said one scientist, who spoke anonymously for fear of losing funding.

Some American grantees have turned to lawyers for advice about how to phrase the responses.

Organizations that are based overseas are most at a disadvantage. The communications pause the Trump administration imposed several weeks ago still prevents them from talking to federal officials who might be able to assist, according to one C.D.C. official.

Grantees are allowed only to talk to American officials in the countries where they are based.

“They could lose the vital funds they need to do their work if they don’t answer these questions appropriately,” said a federal official, who spoke anonymously for fear of retribution.

Gunmen attacked a United Nations helicopter on an evacuation mission in a volatile part of South Sudan on Friday, killing one crew member and injuring two others, the United Nations said.

The helicopter crew was trying to rescue wounded South Sudanese soldiers in Upper Nile State, an attempt to defuse tensions in the area, the United Nations said. It added that the wounded soldiers had also been killed. It did not say whether the helicopter was on the ground or airborne when it came under assault, but one armed group called the White Army said the shooting erupted as the men prepared to board.

The attack followed days of spiking political tensions in the east African nation, the world’s youngest country, amid growing fears among residents and Western officials that it was tumbling toward a new civil conflict.

In a speech appealing for calm, President Salva Kiir of South Sudan vowed not to let that happen. “Let no one take law into their hands,” he said on Friday. “The government which I lead will handle this crisis.”

But critics said that Mr. Kiir and rival leaders were largely responsible for the crisis.

A power-sharing agreement between Mr. Kiir and his first vice president, Riek Machar, ended a brutal civil war in 2018; in recent weeks, it has teetered on the brink of collapse as forces loyal to both men have clashed in Upper Nile State.

South Sudan’s information minister said that 27 soldiers were killed in the helicopter attack on Friday but provided no other detail, and the figure could not be confirmed independently.

The White Army, which is allied with Mr. Machar, issued a statement saying that the dead included a major general of the South Sudan military, three of his bodyguards and the helicopter co-pilot. The general’s guards “opened fire on the White Army security officers who had been assigned to facilitate their safe evacuation,” the statement said, and the White Army returned fire. That account could not be confirmed either.

In Juba, the country’s capital, Mr. Kiir has sought to consolidate his power by arresting Mr. Machar’s allies, including the petroleum minister, Puot Kang Chol, and the deputy army chief, Gen. Gabriel Duop Lam.

The White Army overran an army base at Mr. Machar’s stronghold in Upper Nile State.

“South Sudan is slipping rapidly toward full-blown war,” said Alan Boswell, a South Sudan expert at the International Crisis Group. He warned that any renewed conflict was likely to merge with the war in neighboring Sudan, to devastating effect.

“The region could slip into one large proxy war,” he said in a statement, urging regional powers like Kenya and Ethiopia “to act with supreme urgency” to mitigate the crisis. Otherwise, he warned, “large-scale ethnic massacres” were an imminent threat.

Pope Francis urged Mr. Kiir and Mr. Machar to resolve their differences during a visit to South Sudan two years ago. But the latest cycle of arrests and violence has stoked fears that the relationship is crumbling beyond repair.

South Sudan’s minister for peace-building, Stephen Par Kuol, was among those arrested by Mr. Kiir’s loyalists this week, though he was released on Friday.

South Sudan was formed in 2011 when it split from Sudan after a brutal decades-long struggle for independence. But within a few years, a bitter rivalry between Mr. Kiir and Mr. Machar led to a bloody civil war that eventually caused an estimated 400,000 deaths.

Peace, always shaky, has been severely tested in the past two years as the civil war in Sudan, to the north, has wounded South Sudan’s weak economy by cutting the oil production that produces most of the country’s revenue. Presidential elections, which have been postponed repeatedly, are now scheduled to take place next year.

The United States, which played a central role in the creation of South Sudan, is the country’s largest international donor. But the Trump administration, which is slashing America’s foreign aid budget, has identified South Sudan as a place where U.S. presence should be scaled back.

Frantic international efforts to calm the combustible political situation received a hard blow on Friday as news of the helicopter assault emerged.

Nicholas Haysom, the chief of the United Nations mission to Sudan, condemned the attack as “utterly abhorrent” and said it “may constitute a war crime under international law.”

A Pentagon agency said on Friday that it had “temporarily suspended” the sharing of satellite imagery with Ukraine in accordance with orders from President Trump, the latest in a series of steps taken by the administration to pressure Kyiv into peace talks with Russia.

The move, by the National Geospatial-Intelligence Agency, could shut off a vital window on the war for Ukraine, which has used satellite images purchased by the United States to track the movements of Russian troops and to assess damage to infrastructure on both sides of the long front line, according to a Ukrainian defense news agency.

The Trump administration has also halted military aid and stopped sharing intelligence with Ukraine after a contentious White House meeting last week with the country’s president, Volodymyr Zelensky. The end of American assistance, analysts said, could reorder the battlefield and potentially give Russia a decisive advantage.

President Trump has so far ignored Russia’s role as the aggressor in the war in Ukraine, but on Friday, he said that he was “strongly considering” imposing new sanctions and tariffs on Russia until a cease-fire and permanent peace deal was reached. His message came after Russian forces unleashed a massive bombardment aimed at critical Ukrainian infrastructure overnight on Thursday and on Friday.

Maxar Technologies, a Colorado-based company that has provided satellite images to Ukraine through a Pentagon program, confirmed the suspension of imagery sharing in a statement, emphasizing that it abides by the “contractual commitments” it makes with customers like the United States and other countries.

Warning that a “profound change of American geopolitics” had put Poland, as well as Ukraine, in an “objectively more difficult situation,” Prime Minister Donald Tusk of Poland on Friday said his country must drastically increase the size of its military and even “reach for opportunities related to nuclear weapons.”

Mr. Tusk, in a detailed speech on security to the Polish Parliament, did not explicitly propose developing a nuclear arsenal, but said that “it is time for us to look boldly at our possibilities of having the most modern weapons” and explore options for nuclear and “modern unconventional weapons.”

He added that his government was “talking seriously” with France, Europe’s only nuclear power aside from Britain and Russia, about the possibility of extending the French nuclear umbrella to other European countries. In addition, he said, Poland needs to ensure that all adult men are “trained in the event of war.”

His comments reflect the stark turnabout at the White House with the return of President Trump, who has publicly denigrated the NATO alliance, cast doubt on the U.S. commitment to defend much of Europe in the event of a Russian attack and falsely shifted the blame for Russia’s invasion of Ukraine onto Kyiv. Mr. Trump’s views were received with deep alarm in Warsaw and other parts of Eastern Europe that have bitter memories of being bullied and repeatedly occupied by Russia over centuries.

Poland is a signatory to the Nuclear Nonproliferation Treaty, which prohibits countries that were not among five declared nuclear powers in 1970, when the treaty took effect, from acquiring atomic weapons.

But Israel, India and Pakistan, which never signed onto the ban, and North Korea, which withdrew from the treaty, have all developed nuclear weapons, and there has been sporadic discussion in Poland of trying to join the nuclear club. Jaroslaw Kaczynski, the leader of Poland’s former governing party, said in 2022 that he would, “as a citizen,” like to see Poland acquire nuclear weapons but added: “As a responsible politician, I must assess this idea as unrealistic.”

Russian officials have suggested repeatedly that they could use nuclear arms in the fight against Ukraine, which is not a NATO member, particularly if the West ramps up its military aid to Ukraine. But repeated escalations in that aid so far have not prompted such a response.

The French Institute of International Relations warned in a report last year that “the war in Ukraine has the potential to increase proliferation risks, as it signals that nuclear powers can attack an adversary with conventional capabilities while backing its actions with nuclear threats to deter third-party intervention.”

“The war also sends the message that nuclear weapons are a necessary guarantor of national security,” the report said.

When the Soviet Union fell apart in 1991, Ukraine held on its territory the world’s third-largest nuclear arsenal but relinquished it in 1994, in return for a pledge from Russia, as well as the United States and Britain, to respect Ukrainian borders and refrain from the use or threat of military force. Many Ukrainians now regret giving up this arsenal, as their government has appealed in vain for NATO membership and direct involvement of Western forces in the war.

Poland is the biggest military power among former members of the Soviet-led Warsaw Pact military alliance that are now members of NATO. But those nations have long looked to the United States, with its nuclear arsenal and thousands of troops stationed in Poland and elsewhere in Eastern Europe, as the guarantor of its security.

Poland has a long tradition of nuclear expertise, dating to Marie Sklodowska-Curie, the Polish-born French scientist who won Nobel Prizes in physics and chemistry for her pioneering work in discovering radioactivity — a word she coined — and radioactive elements in the early 20th century. Stanislaw Ulam, a Polish mathematician and physicist, played an important role in the Manhattan Project, the United States’ secret atomic bomb program during World War II, and in the subsequent invention of the hydrogen bomb.

Under Communism, Poland trained a large group of nuclear engineers in preparation for a nuclear power plant that it started building with Soviet assistance but never finished. Poland last year approved plans for the construction of its first atomic power station under a contract with America’s Westinghouse Electric.

Mr. Tusk dedicated most of his speech to the new security situation created by Mr. Trump’s abrupt upending of what had been the pillars of American foreign policy since the end of World War II.

“We cannot deny these facts: Today, Poland’s situation, objectively, and Ukraine’s situation, in particular, is more difficult than it was a few months ago, and we must deal with this fact,” he said.

“Washington’s much more symmetrical attitude toward Moscow and Kyiv, much more symmetrical than we had become accustomed to, is a little different — I am also putting it mildly — from what we feel in Poland or in Europe,” he said.

But Mr. Tusk avoided criticism of Mr. Trump and said the “closest possible ties” with the United States remained essential.

At the same time, he said Poland would expand its military to around half a million personnel, including reservists, more than double its current size and raise spending on defense to 5 percent of its economic output.

But Mr. Tusk ruled out sending Polish soldiers to Ukraine “as part of some contingent,” an apparent reference to French proposals that European countries provide troops for a future security force if U.S. efforts to broker a peace deal bear fruit.

Poland is already one of Europe’s biggest spenders on defense, spending around 4 percent of gross domestic product last year, double the 2 percent minimum set by NATO for its member nations. Most NATO countries are above that threshold now, unlike during Mr. Trump’s first term, when he berated them for not spending more, but he continues to question the U.S. obligation to defend its allies.

Poles, Mr. Tusk said, “will not accept the philosophy that we are powerless and helpless, that if President Trump decides to change his policy, we have no chance.”

“I will repeat once again what seems incredible but is true: 500 million Europeans are begging 300 million Americans to protect us from 180 million Russians who have not been able to cope with 40 million Ukrainians for three years,” he said.

Europe has the means to defend itself but needs to “eliminate one important deficit,” he added — “the lack of will to act, uncertainty and, sometimes, even cowardice.”

Marc Santora contributed reporting from Kyiv, Ukraine.