It was not entirely clear from Mr. Rubio’s remarks whether he meant that the United States would merely abandon its effort to reach a 30-day cease-fire between Russia and Ukraine, President Trump’s immediate focus, or abandon Washington’s commitments to Ukraine altogether.

But his remarks were certain to worry Ukraine, which is heavily dependent on American military support, and appeared intended to inject urgency into European efforts to prod Volodymyr Zelensky, the Ukrainian president, toward painful compromise. Later on Friday, Mr. Trump vented his own frustrations at the way at least one country had failed to respond to American overtures.

“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 just going to take a pass,” Mr. Trump said from the Oval Office. “But hopefully we won’t have to do that.”

Mr. Trump said the war was costing the lives of 2,500 soldiers every week but has put virtually no pressure on Russia to end the fighting and at one point claimed that Ukraine was responsible for the Russian invasion in 2022.

While the United States is Russia’s chief interlocutor in the negotiations, Europe has far greater sway over Mr. Zelensky. Mr. Trump said on Thursday that he was “not a big fan” of the Ukrainian leader. It is clear, not least in Kyiv, where Mr. Trump’s sympathies lie.

As Mr. Trump and Mr. Rubio voiced their impatience, Vice President JD Vance sounded a different note on a visit to Rome with his family. Speaking before a meeting with Giorgia Meloni, the right-wing prime minister who is a favorite of the Trump administration for her conservative views, he said he was “optimistic” about the negotiations to end the fighting in Ukraine.

“I won’t prejudge them, but we do feel optimistic that we can hopefully bring this war, this very brutal war, to a close,” he said.

Mr. Trump’s approach to governance often appears to place a lower priority on coordination and planning between members of the cabinet than on keeping interlocutors guessing through seemingly scattershot statements. This appeared to be another instance of that practice.

Mr. Rubio said Mr. Trump “has spent 87 days at the highest level of this government repeatedly making efforts to bring this war to an end. We are now reaching a point where we need to decide and determine whether this is even possible or not.”

Before taking office, Mr. Trump said he would end the war within “24 hours.”

Responding to Mr. Rubio’s comments, the Kremlin signaled that it was in no hurry for a cease-fire, a consistent message from Moscow throughout Mr. Trump’s attempts to end the war.

High-level talks on Thursday between American, European and Ukrainian officials were the first of their kind, intended to bring “convergence” between views of the war in Washington and European capitals. Mr. Rubio said the conversations had been constructive, but it appeared clear that Mr. Trump was losing patience.

During the talks, a “broad framework” for peace, in effect an American plan, was presented to Ukraine. “It’s a framework that gets us into a position to see — look, there are going to be differences; there’s no — no one’s saying this can be done in 12 hours,” Mr. Rubio said. He declined to give any further details of the proposal.

“It is not our war. We didn’t start it,” Mr. Rubio said. “The United States has been helping Ukraine for the past three years and we want it to end, but it’s not our war.”

He added: “If it’s not possible — if we’re so far apart that this is not going to happen — then I think the president’s probably at a point where he’s going to say, well, we’re done. We’ll do what we can on the margins.” He described Mr. Trump as feeling “very strongly” about this.

Mr. Rubio and Steve Witkoff, Mr. Trump’s envoy, have led the American diplomacy aimed at ending the war, which has festered for more than three years. Mr. Witkoff has met with Mr. Putin multiple times and said he was trying to develop a “friendship, a relationship” with the Russian leader.

But Mr. Putin has balked, setting various conditions even for a 30-day cease-fire. The Russian bombardment of Ukraine continues.

Mr. Rubio’s comments came after he spoke with Sergey V. Lavrov, the Russian foreign minister, on Thursday. In that call, the State Department said, Mr. Rubio told his Russian counterpart that “peace is possible if all parties commit to reaching an agreement,” and that the United States had been encouraged by the Ukrainian and European response in Paris to Mr. Trump’s plan for peace.

The Russian Foreign Ministry’s description of that call was more circumspect. Russia said that Mr. Lavrov “reaffirmed Moscow’s readiness to continue collaborative efforts with American counterparts to comprehensively address the root causes of the Ukrainian crisis.”

Asked whether Russia planned to respond to a cease-fire offer from Mr. Trump this week, Dmitri S. Peskov, the Kremlin’s spokesman, said on Friday that ending the war was “not a simple topic” and that Russia was seeking a settlement that would “ensure its own interests.”

“We are open for dialogue,” Mr. Peskov said. “There are already certain developments, but many difficult discussions lie ahead.”

There was no immediate official reaction from Kyiv to Mr. Rubio’s remarks, which appeared jolting after strained efforts on Thursday to demonstrate harmony between American and European approaches to the war.

Mykhailo Samus, the director of the New Geopolitics Research Network in Kyiv, said that an American exit from peace talks would mean an open acknowledgment of Mr. Trump’s “powerlessness regarding the Russia-Ukraine war.”

He added that, “this calls into question the signing of an agreement on strategic resources between the U.S. and Ukraine.”

Ukraine and the United States signed a memorandum of understanding late on Thursday as a “step toward a joint economic partnership agreement,” according to Ukraine’s economy minister, bringing both sides closer to a contentious minerals deal. The deal, if concluded, would make American abandonment of Ukraine appear a remote possibility.

The officials who took part in the Paris talks have agreed to meet next week in London, and Mr. Rubio indicated he might participate, but not if the meeting was just an exercise in further talking. He said he hoped Europeans would remain engaged in the efforts to secure a peace in Ukraine. Mr. Putin’s demands — among them that Ukraine cede territory that Russia has occupied and abandon its attempts to join NATO — have been rejected outright by Ukraine.

“I think the U.K. and France and Germany can help us move the ball on this and then get this closer to a resolution,” Mr. Rubio told reporters at Le Bourget airport as he prepared to depart. “I thought they were very helpful and constructive with their ideas.”

Anton Troianovski and Maria Varenikova contributed reporting.