Top U.S. and Ukrainian officials said Sunday they were making progress toward ending the Russia-Ukraine war but provided scant details as they discussed the American proposal 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 said high-stakes talks in Geneva were “very worthwhile” and constituted the most productive day in “a very long time” on larger efforts to end the fighting.

“I feel very optimistic that we can get something done," Rubio said.

But he offered very little information on what was discussed, allowing only that discussions would continue on Monday and perhaps deeper into the week. He also suggested that Europe's role and responsibilities as spelled out under the U.S. peace plan might be separated from facets more directly involving the responsibilities of Russia and Ukraine.

And Rubio noted that any final product, once it's ready, will still have to be presented to Moscow: “obviously, the Russians get a vote here."

Rubio's attempt to strike such a positive tone while offering no evidence of a breakthrough constituted a more nuanced approach than Trump himself.

Before talks even really began, the president blasted Ukraine for a lack of gratitude for U.S. military assistance, while shying away from criticizing Russia.

Trump has set a Thursday deadline for Ukraine to respond to the plan.

But he also has suggested that could slide if there was proof of real progress and that the plan was not his final offer.

Rubio also undermined the idea that the deadline was looming, saying simply that officials want to see fighting stop as soon as possible.