Washington and Kyiv appear to have made notable shifts to secure progress but huge questions remain, not least whether Moscow will once again reject making concessions.

Debris from a Russian attack in Kramatorsk, Ukraine, on Tuesday.Jose Colon / Anadolu via Getty Images

President Donald Trump says that Russia and Ukraine are closer to peace than they have ever been.

Days of intense negotiations ended with the United States, Ukraine and Europe hailing significant progress as Trump pushes for a deal to end the Kremlin`s war by Christmas.

Washington and Kyiv appear to have made notable shifts to secure progress but huge questions remain, not least whether Moscow will once again flatly reject the idea of making any significant concessions.

And while the U.S. has now offered Ukraine the robust security guarantees it has long sought, the fate of key territory on the conflict`s eastern front lines remains pivotal, yet unresolved.

After meetings with Trump`s special envoy Steve Witkoff, son-in-law Jared Kushner and European leaders in Berlin, Ukrainian President Volodymyr Zelenskyy told reporters Monday that Kyiv had now been offered an equivalent of NATO`s Article 5 deterrence mechanism, under which an attack on one member is considered an attack on them all.

This guarantee would be legally binding, he said, voted on and approved by the U.S. Congress.

Zelenskyy said the latest draft was not "perfect" but was "very workable." He abandoned Ukraine`s long-held ambition of joining NATO over the weekend, ahead of the talks.

These guarantees are "the biggest win so far for Ukraine and for Europe," a U.S. official told reporters, warning that they "will not be on the table forever." The official added that they believe Russia will accept it in a final deal.

Zelenskyy has maintained that any deal would need to provide concrete security guarantees to prevent future attacks by Russia. It`s a historically sensitive issue for Kyiv since Moscow annexed the Crimean Peninsula in 2014 in violation of the Budapest Memorandum, meant to guarantee Ukraine`s post-Soviet sovereignty in exchange for giving up its nuclear weapons.

European leaders suggested their countries could lead a "multinational force" supported by the U.S. that would "assist in the regeneration of Ukraine`s forces, in securing Ukraine`s skies, and in supporting safer seas," alongside a U.S.-led ceasefire monitoring, verification and deconfliction mechanism.

All of this was discussed on a call with Trump, Zelenskyy said, and the American leader appeared happy with the progress.

"I think we`re closer now than we have been ever," Trump said Monday.

The White House did not immediately respond to NBC News request for further comment on the status of the talks.

But the Kremlin is unlikely to share in the enthusiasm.

"I think that it`s highly unlikely that Putin will buy into this," said Christopher Tuck, an expert in conflict and security at King`s College London. "The draft agreement includes clauses that stand in diametric opposition to policy statements made repeatedly by Russia on what it regards as an acceptable basis for resolving the conflict."

The multinational force is "a clear red line for Putin," said John Lough, head of foreign policy at the New Eurasian Strategies Centre, a London/Washington-based think tank focused on Russia. The Kremlin will also reject the idea of a U.S.-led ceasefire monitoring and verification mechanism, he said, and will likely ask for restrictions on the kinds of weapons systems available to Kyiv`s forces.

Russian Deputy Foreign Minister Sergei Ryabkov told ABC News on Monday that while he thinks Russia and the U.S. might be "on the verge" of a deal, the agreement can`t include the presence of foreign troops in Ukraine.

"So this is likely to go a few more rounds," Lough said.

And still to be worked out is the future of eastern Ukraine.

Putin insists Russia is winning on the battlefield, giving him no incentive to negotiate what he thinks he can achieve by force.

He wants Ukraine to give up its entire Donbas region, even though Kyiv`s forces still control a part of it and it is something Zelenskyy has said he will never do.

"The Americans are trying to find a compromise," Zelenskyy said Monday. "They are proposing a ‘free economic zone` (in the Donbas). And I want to stress once again: a ‘free economic zone` does not mean under the control of the Russian Federation."

Trump has insisted that Ukraine would have to cede territory, but it`s unclear how this compromise would work or whether the Kremlin would give it the time of day.

A U.S. official said they had given Zelenskyy some "thought-provoking ideas," and that they would speak with Russia and Europe about them after his response. Zelenskyy said the U.S. would take the new draft to Russia in the coming days.

Asked if any of the security guarantees floated Monday were unacceptable to the Kremlin, spokesman Dmitry Peskov said Russia had yet to see any updated agreements. He reiterated that Russia wants a comprehensive peace deal, not a short-term ceasefire over Christmas as Kyiv has suggested.

"We want peace. We don`t want a truce to give Ukraine a breathing space and prepare for a continuation of the war," Peskov told reporters.

But while the Kremlin is likely to stonewall the newest agreements, Tuck said, the proposed security guarantees put pressure on Russia by implying a growing consensus between Ukraine and the West on what a peace agreement should look like.

"The agreement isn`t nothing, we aren`t back to square one: the statement does indicate a more united front by the West, and getting the U.S. to commit, even in general and early terms, to providing some form of security guarantees is a significant development," Tuck said.