“We will only end this devastating war and establish a durable peace by coupling allied strength with a realistic assessment of the battlefield,” he said.

Hours later, Mr. Trump wrote on social media that he had a long phone call with President Vladimir V. Putin of Russia, and they had agreed to “start negotiations immediately” to end the war.

Mr. Hegseth, at NATO headquarters in Brussels, said that Mr. Trump expected Europe to bear more financial and military responsibility for Ukraine’s defense. He is scheduled to take part in a meeting of NATO defense ministers there on Thursday.

Europe, he said, must take more responsibility for its conventional defense and spend more money on its armed forces, up to 5 percent of national output, as the United States deals with its own security risks and the challenge of China.

Mr. Trump, he added, does not support Ukraine’s membership in NATO as part of a realistic peace plan.

A defensive line in the Kharkiv region of Ukraine, in December.Credit…Mauricio Lima for The New York Times

After a settlement, “a durable peace for Ukraine must include robust security guarantees to ensure that the war will not begin again,” but that would be the responsibility, he said, of European and non-European troops in a “non-NATO mission” unprotected by NATO’s Article Five commitment to collective defense.

No American troops will be deployed to Ukraine, he said, and Europe should provide “the overwhelming share of future lethal and nonlethal aid to Ukraine.”

European and alliance leaders have been anxiously waiting to hear what Mr. Trump’s goals for a Ukraine settlement might be. Mr. Hegseth’s comments, at the opening of a meeting of the Ukraine Defense Contact Group — more than 50 nations, including all 32 NATO member states — will not come as a great surprise.

But they mark a major change from former President Biden’s policy that it was up to Ukraine to decide whether to make concessions in exchange for peace — and so far, Ukraine has made no concessions on its claim to sovereignty within its internationally recognized borders.

Mr. Trump said this week that he would trade continued aid to Ukraine for some $500 billion in Ukrainian rare earth minerals used in high-tech manufacturing. He has sent Scott Bessent, the new treasury secretary, to make the administration’s first high-level visit to Kyiv, because, Mr. Trump wrote on social media, “this war must and will end soon.”

NATO has promised that Ukraine will one day become a member of NATO, but without specifying a date. Mr. Hegseth’s comments would appear to put that date very far into the unforeseeable future, if it arrives at all.

His remarks will create political difficulties for President Volodymyr Zelensky of Ukraine and are likely to please Mr. Putin, who seized Crimea from Ukraine in 2014, and launched an all-out invasion in 2022. Russia now occupies about 20 percent of Ukraine.

Mr. Putin has demanded that Russia keep its occupied territories and take control of additional land it claims, that Ukraine not join NATO, that Ukraine’s military capacity be limited and that NATO enlargement halt. He has said he is willing to join negotiations on a settlement with Ukraine, but only on his terms.

To help bring Mr. Putin to the negotiating table, Mr. Hegseth urged lower energy prices, “coupled with more effective enforcement of energy sanctions.”

“The real conversation has started,” said Camille Grand, a former NATO assistant secretary general. After quiet hints from Trump officials, Mr. Hegseth “has now presented the American ask and the terms are clear,” Mr. Grand said. “Now the Europeans need to respond.”

“Either the Europeans say, ‘Oh my god, we can’t do this without you Americans,’ and add to the Trump perception that they are useless and security free-riders,” he said, “or more likely, ‘We’re ready to look into this and mobilize troops and resources, but these are our conditions to do it.’”

Ukrainian national flags flutter over the graves of dozens of soldiers killed in the war with Russia, at a cemetery in the city of Vasylkiv, Ukraine in November.Credit…Mauricio Lima for The New York Times

Then there can be a serious conversation about the solidity of any cease-fire, about a peacekeeping force, command and control, air cover, and worst-case scenarios, if Russia tests the peacekeepers, Mr. Grand said.

Mr. Hegseth did not question American commitment to NATO, as President Trump has sometimes done in the past.

The United States “remains committed to the NATO alliance and to the defense partnership with Europe, full stop, but the United States will no longer tolerate an imbalanced relationship which encourages dependency,” Mr. Hegseth said.

So Europe must step up to take responsibility for its own conventional defense, he said, while implying that the American nuclear umbrella that helps protect NATO and Europe would remain in place.

He urged Europeans to keep their commitments on military spending and increase them. “We challenge your countries and your citizens to double down and recommit yourselves not only to Ukraine’s immediate security needs, but to Europe’s long term defense and deterrence goals,” he said.

“We hear you,” said John Healey, Britain’s defense secretary, in response to Mr. Hegseth’s remarks before the meeting was closed to the media.

Speaking at a news conference after the meeting, Mr. Healey pointed to shared goals with the United States, including a durable peace in Ukraine with security guarantees, and increased European military spending and responsibility both for Ukraine and for its own defense.

Asked if Mr. Trump had now broken Western unity on Ukraine, Mr. Healey did not answer but repeated these shared goals, emphasizing the American commitment to NATO and refusing to rule out Ukraine joining NATO one day.

“That is a process that will take some time,” he said. But for now, he said, “the duty of the nations around that table is to make sure that Ukraine is in the strongest possible position going into any talks in the future.”

As for Europe taking responsibility for the bulk of aid to Ukraine, he noted that Europe is already providing more aid in total than the United States. “We are stepping up the support for Ukraine,” Mr. Healey said. “We will do more. We’ll do that alongside the Americans.”