New York. Russian leader Vladimir Putin agreed at his summit with President Donald Trump that the United States and its European allies could offer Ukraine a security guarantee resembling NATO’s collective defense mandate as part of an eventual deal to end the war, a US official said Sunday.

Trump envoy Steve Witkoff, who took part in the talks Friday at a military base in Alaska, said it “was the first time we had ever heard the Russians agree to that” and called it “game-changing.”

“We were able to win the following concession: that the United States could offer Article 5-like protection, which is one of the real reasons why Ukraine wants to be in NATO," Witkoff told CNN's “State of the Union.”

Witkoff offered few details on how such an arrangement would work. But it appeared to be a major shift for Putin and could serve as a workaround to his deep-seated objection to Ukraine’s potential NATO membership, a step that Kyiv has long sought.

It was expected to be a key topic on Monday as Ukrainian President Volodymyr Zelensky and major European leaders meet with Trump at the White House to discuss ending the 3 1/2-year conflict.

“BIG PROGRESS ON RUSSIA,” Trump said Sunday on social media. “STAY TUNED!”

On Sunday night, however, Trump seemed to put the onus on Zelensky to agree to concessions.

“President Zelensky of Ukraine can end the war with Russia almost immediately, if he wants to, or he can continue to fight," he wrote. "Remember how it started. No getting back Obama given Crimea (12 years ago, without a shot being fired!), and NO GOING INTO NATO BY UKRAINE. Some things never change!!!"

Hammering Out a Plan for Security Guarantees
Article 5, the heart of the 32-member transatlantic military alliance, says an armed attack against a member nation is considered an attack against them all.

What needed to be hammered out at this week's talks were the contours of any security guarantees, said Secretary of State Marco Rubio, who also participated in the summit. Ukraine and European allies have pushed the United States to provide that backstop in any peace agreement to deter future attacks by Moscow.

“How that’s constructed, what we call it, how it’s built, what guarantees are built into it that are enforceable, that’s what we’ll be talking about over the next few days with our partners," Rubio said on NBC’s “Meet the Press.”

It was unclear, however, whether Trump had fully committed to such a guarantee. Rubio said it would be “a huge concession."

The comments shed new light on what was discussed in Alaska. Before Sunday, US officials had offered few details even as both Trump and Putin said their meeting was a success.

Witkoff also said Russia had agreed to enact a law that it would not “go after any other European countries and violate their sovereignty.”

“The Russians agreed on enshrining legislatively language that would prevent them from -- or that they would attest to not attempting to take any more land from Ukraine after a peace deal, where they would attest to not violating any European borders," he said on “Fox News Sunday.”

Europe Welcomes US Openness to Security Guarantees
European Commission President Ursula von der Leyen, speaking in Brussels alongside Zelensky, applauded the news from the White House as a European coalition looks to set up a force to police any future peace in Ukraine.

"We welcome President Trump’s willingness to contribute to Article 5-like security guarantees for Ukraine and the ‘coalition of the willing’ -- including the European Union -- is ready to do its share,” she said.

Zelensky thanked the US for signaling that it was willing to support such guarantees but said much remained unclear.

“There are no details how it will work, and what America’s role will be, Europe’s role will be and what the EU can do -- and this is our main task: We need security to work in practice like Article 5 of NATO,” he said.

French President Emmanuel Macron said the substance of security guarantees to secure any peace arrangement will be more important than whether they are given an Article 5-type label.

At the White House meeting, Macron said European leaders will ask the US to back their plans to beef up Ukraine’s armed forces with more training and equipment and deploy an allied force away from the front lines.

“We’ll show this to our American colleagues, and we’ll tell them, ‘Right, we’re ready to do this and that, what are you prepared to do?’" Macron said. “That’s the security guarantee.”

Defending Trump's Shift from Ceasefire to Peace Deal
Witkoff and Rubio defended Trump’s decision to abandon a push for a ceasefire, arguing that the Republican president had pivoted toward a full peace agreement because so much progress had been made at the summit.

“We covered almost all the other issues necessary for a peace deal,” Witkoff said, without elaborating. “We began to see some moderation in the way they’re thinking about getting to a final peace deal.”

Rubio, appearing on several TV news shows Sunday, said it would have been impossible to reach any truce Friday because Ukraine was not there.

“Now, ultimately, if there isn’t a peace agreement, if there isn’t an end of this war, the president’s been clear, there are going to be consequences,” Rubio said on ABC’s “This Week.” “But we’re trying to avoid that."

Rubio, who is also Trump's national security adviser, also voiced caution on the progress made.

“We’re still a long ways off," he said. “We’re not at the precipice of a peace agreement. We’re not at the edge of one. But I do think progress was made towards one.”