After wrapping up what he described as a productive round of talks toward ending the war in Ukraine hosted in Paris, Secretary of State Marco Rubio on Friday laid out a surprising ultimatum for all parties involved, declaring that the U.S. was willing to move on from the negotiations if they didn't yield results soon.

"We need to figure out here, now within a matter of days, whether this is doable in the short term, because if it's not, then I think we're just going to move on," Rubio said on the tarmac as he left France early Friday morning.

"We have other priorities to focus on as well," he added. "It's not our war."

Later, at the White House, President Donald Trump echoed Rubio's assertion -- saying that the U.S. would make a determination "very shortly."

"Now, if for some reason, one of the two parties makes it very difficult, we're just going to say you're foolish, you're fools, you're horrible people, and we're going to just take a pass," Trump said without specifying which party. "But hopefully, we won't have to do that."

Trump also struck an optimistic note, and declined to say whether he would in fact walk away from the talks entirely or if he believed Russian President Vladimir Putin was stalling.

"My whole life has been one big negotiation. And I know when people are playing us and I know when they're not, and I have to see an enthusiasm to want to end it. And I think I see that enthusiasm," he said. "I think I see it from both sides."

While the president declined to set a specific deadline for progress, Rubio looked ahead to a new round of talks slated to take place in London early next week as a potentially decisive engagement, saying he may attend -- but only if the meeting appeared poised to result in action rather than just talk.

The Trump administration's apparent ultimatum comes as negotiators have grown increasing frustrated over the lack of momentum, especially from the Russian side.

Officials say that the Kremlin has repeatedly insisted on reprieve from economic sanctions in exchange for taking incremental steps toward peace. Relief for Moscow would require action from both the U.S. and its allies, and so far, it appears to be a step that European leaders are unwilling to take.

"I always remind everybody that part of the sanctions against Russia, many of them are European sanctions that we can't lift," Rubio said.

Rubio sought to ramp up the pressure on Moscow during a call with Russian Foreign Minister Sergey Lavrov on Thursday, according to officials familiar with the conversation, who add that the administration was hopeful progress could be made in the hours between their talk and the secretary's departure from France.

But in the wake of Rubio's public comments, Moscow signaled it was in no hurry to strike a deal.

"We believe that certain progress can, in fact, already be acknowledged," Kremlin spokesman Dmitry Peskov said. "There are already some developments, but, of course, there are still many complicated discussions ahead."

These developments come as one potential indicator of progress -- a 30-day ceasefire intended to pause strikes on energy infrastructure targets—has expired, with no word from Putin on whether Moscow will restart attacks on those targets.

Both Russia and Ukraine have accused each other of repeatedly violating the agreement, which was brokered by the Trump administration last month.

While it's unclear if Rubio and Trump's comments will have any impact on Moscow, they are likely to stoke anxiety in Kyiv. So far administration officials have given no indication that the U.S. would reup support for Ukraine's war effort if it abandons peace negotiations.

"Although the Trump administration is right to act with a sense of urgency, threatening to wash our hands of the peace talks could perversely incentivize Russian intransigence since Moscow will likely calculate that the United States walking away would lead to an end to American aid for Ukraine," said John Hardie, the deputy director of the Foundation for Defense of Democracy's Russia Program.

"Instead, Washington should focus on putting pressure on Russia, including by tightening sanctions on Russian oil revenue," Hardie continued. "Meanwhile, Washington should strive to help Ukrainian forces exhaust Russian offensive potential to show Putin he cannot gain through further fighting."

If the U.S. does in fact withdraw from negotiations, Rubio said only that members of the Trump administration would "do what we can on the margins."