President Volodymyr Zelensky of Ukraine responded cautiously on Sunday morning to Russia’s offer of direct talks, saying it was a “positive sign” but pressing for an immediate truce first, just hours after President Vladimir V. Putin of Russia had rejected a cease-fire.

Mr. Zelensky’s response did not directly address Mr. Putin’s suggestion that representatives of the two countries meet on May 15week in Istanbul. But the Ukrainian leader said in a social media post that he “expected Russia to confirm a cease-fire” that would start on Monday.

The back and forth between Mr. Zelensky and Mr. Putin came after the leaders of France, Britain, Germany and Poland visited Kyiv and announced a Monday deadline for Russia to accept a 30-day cease-fire or face additional sanctions. The European leaders, who spoke by phone from Kyiv on Saturday with President Trump, said that the United States would also impose sanctions on Russia if Mr. Putin did not agreed to a truce.

Since initiating efforts to end the war in Ukraine three months ago, the Trump administration has aligned with Russia on a vote in the United Nations on the war and resumed broader diplomatic ties with Moscow. It has left in place existing economic sanctions but has not ramped up pressure with additional restrictions, even as Mr. Putin has shown little sign of pulling back in the war. By contrast, Ukraine agreed in March to an American proposal for an unconditional cease-fire.

At a late-night news conference on Saturday, Mr. Putin responded to the European deadline by asking for direct talks with Ukraine that would begin in Istanbul in the coming days, but he did not accept the cease-fire proposal. Earlier, the Kremlin spokesman said Russia would do so only if Western governments stopped supplying arms to Ukraine.

Mr. Trump praised the diplomatic efforts in a social media post overnight, saying that it was a “potentially great day for Russia and Ukraine!” But the president did not give any indication of how he viewed Mr. Putin’s response, or whether it would trigger new American sanctions on Russia.

“Think of the hundreds of thousands of lives that will be saved as this never ending ‘bloodbath’ hopefully comes to an end,” he wrote. “It will be a whole new, and much better, WORLD. I will continue to work with both sides to make sure that it happens.”

President Emmanuel Macron of France called Russia’s response insufficient, saying as he left Ukraine that it was “a first step but not enough.” Mr. Macron said that Mr. Putin was stalling with his counteroffer of direct talks and “wants to buy time.”

The European leaders who visited Kyiv on Saturday were clear that only with an unconditional cease-fire would Moscow avoid coming under additional sanctions.

The Kremlin had declared a unilateral, three-day truce last week around the country’s celebration of the anniversary of victory over Nazi Germany. Ukraine’s military confirmed that most long-range strikes had halted over that period, but fighting along the front carried on without pause.

Russia resumed its long-range drone attacks on Ukraine overnight on Sunday. From midnight until mid-morning Sunday, Russian forces launched 108 Iranian-designed Shahed drones, the Ukrainian air force said, setting off air alarms in several cities.