President Trump made an unusually sharp appeal to President Vladimir V. Putin of Russia on Thursday, calling on him to stop his bombing campaign in Ukraine and agree to a peace deal after the deadliest attack on Kyiv in nearly a year.

“Not necessary, and very bad timing. Vladimir, STOP! 5000 soldiers a week are dying. Let’s get the Peace Deal DONE!” Mr. Trump wrote on social media.

Russia’s missile attack came a day after the Trump administration threatened to abandon peace talks if Ukraine did not accept a U.S. peace proposal that heavily favored Russia.

Mr. Trump’s comments were striking because he has mostly avoided even mild criticism of Mr. Putin in his handling of the talks so far. Instead, he has directed most of his anger toward President Volodymyr Zelensky of Ukraine, calling him a “dictator” and describing him as the main impediment to a peace deal.

While Mr. Trump made clear he was running out of patience for the two sides to agree on a peace deal, he also sought to pre-emptively divert blame should the negotiations fall apart, a sign that he is perhaps more pessimistic than he was when he regained the presidency brimming with confidence about his talent as a negotiator.

The Russia-Ukraine war, which Mr. Trump had previously said he could resolve in “24 hours,” was now, he suggested, a matter of great difficulty and complexity.