Top U.S. and Ukrainian officials said Sunday they were making progress toward ending the Russia-Ukraine war as they discussed a U.S.-proposed plan to achieve peace despite concerns among many of Washington's European allies that the plan was far too conciliatory to Moscow.

U.S. Secretary of State Marco Rubio described the first session of high-stakes talks in Geneva as "probably the most productive and meaningful meeting" since President Trump returned to office in January. 

The White House said in a statement Sunday night that U.S. and Ukrainian officials "drafted an updated and refined peace framework" following their discussions.

"They reaffirmed that any future agreement must fully uphold Ukraine's sovereignty and deliver a sustainable and just peace," the statement read in part. "Both sides reiterated their readiness to continue working together to secure a peace that ensures Ukraine's security, stability, and reconstruction."

Ukrainian President Volodymyr Zelenskyy said in a post on Telegram that the delegates had "substantive conversations" that would continue.

"It is important that there is dialogue with the American representatives, and there are signals President Trump's team is hearing us," he said. "And it is important to ensure that the steps toward ending the war are effective, and that everything is workable."

Neither Rubio nor Zelenskyy offered specific details on any changes made to the proposed plan. Rubio left Geneva to return to Washington, D.C, following Sunday's talks, according to an official from the State Department.

Earlier, Mr. Trump appeared less optimistic and used a lengthy online post to blast Ukraine for a lack of gratitude for U.S. military assistance, while shying away from criticizing Russia.

In a Truth Social post before Sunday's talks, Mr. Trump said the Ukraine-Russia war is "a violent and terrible one that, with strong and proper U.S. and Ukrainian LEADERSHIP, would have NEVER HAPPENED."

"I INHERITED A WAR THAT SHOULD HAVE NEVER HAPPENED, A WAR THAT IS A LOSER FOR EVERYONE, ESPECIALLY THE MILLIONS OF PEOPLE THAT HAVE SO NEEDLESSLY DIED. UKRAINE 'LEADERSHIP' HAS EXPRESSED ZERO GRATITUDE FOR OUR EFFORTS, AND EUROPE CONTINUES TO BUY OIL FROM RUSSIA," he wrote. "THE USA CONTINUES TO SELL MASSIVE $AMOUNTS OF WEAPONS TO NATO, FOR DISTRIBUTION TO UKRAINE (CROOKED JOE GAVE EVERYTHING, FREE, FREE, FREE, INCLUDING "BIG" MONEY!). GOD BLESS ALL THE LIVES THAT HAVE BEEN LOST IN THE HUMAN CATASTROPHE! President DJT."

Mr. Trump has set a Thursday deadline for Ukraine to respond to the plan. But he also has suggested that he could push that back if there was proof of real progress and that the plan was not his final offer.

The 28-point blueprint drawn up by the U.S. sparked alarm in Kyiv and across Europe, quickly rallying Western allies around Ukraine in a push to revise the plan, which is seen as favoring Moscow. They said in a joint statement on Saturday that while the plan "includes important elements that will be essential for a just and lasting peace," additional work is required. The plan acquiesces to many of Russia's demands that Ukraine has categorically rejected on dozens of occasions, including giving up large pieces of territory.

After the first session, Rubio said, "This will ultimately have to be signed off by our presidents, although I feel very comfortable about that happening given the progress we've made." He was joined at the talks by U.S. Army Secretary Dan Driscoll and Trump's special envoy Steve Witkoff.

Russia will also need to approve the final peace plan, Rubio said.

The head of the Ukrainian delegation, presidential chief of staff Andriy Yermak, also confirmed an initial session of talks had concluded and that a second meeting would soon take place. 

After Mr. Trump's post, Zelenskyy cheered U.S.-led efforts on security, but stressed that "the crux of the entire diplomatic situation is that it was Russia, and only Russia, that started this war."

"Ukraine is grateful to the United States, to every American heart, and personally to President Trump for the assistance that – starting with the Javelins – has been saving Ukrainian lives. We thank everyone in Europe, in the G7, and in the G20 who is helping us defend life. It is important to preserve the support.

"It is important not to forget the main goal – to stop Russia's war and prevent it from ever igniting again."

"The leadership of the United States is important, we are grateful for everything that America and President Trump are doing for security, and we remain as constructive as possible," Zelenskyy said in a post on Telegram.

Some U.S. lawmakers said Saturday that Rubio had privately described the plan as a Russian "wish list" rather than a Washington-led proposal. The bipartisan group of senators held a news conference, stating that they had spoken with Rubio about the peace plan after he reached out to some of them while en route to Geneva. Independent Maine Sen. Angus King said Rubio told them the plan "was not the administration's plan" but a "wish list of the Russians."

Rubio on Saturday night rejected the claims, writing in a social media post that, "The peace proposal was authored by the U.S. It is offered as a strong framework for ongoing negotiations. It is based on input from the Russian side. But it is also based on previous and ongoing input from Ukraine."

State Department spokesperson Tommy Pigott denied their account, calling it "blatantly false."

"As Secretary Rubio and the entire Administration has consistently maintained, this plan was authored by the United States, with input from both the Russians and Ukrainians," he posted on social media.

A senior U.S. official told CBS News in a statement Saturday night that the Trump administration has "consistently maintained" that the peace plan was authored by the U.S. with input from Russia and Ukraine.

"This plan has always been a hopeful start to continued negotiations, and eventually the signing of a final peace agreement once and for all," the statement said.