European leaders stepped up pressure on Russia to accept an unconditional cease-fire in Ukraine, threatening to immediately impose a new round of punishing sanctions if the Kremlin did not change its stance by the end of Monday.

“The clock is ticking — we still have 12 hours until the end of this day,” the German government spokesman, Stefan Kornelius, told a news conference.

The ultimatum was the latest turn in an increasingly frenetic round of diplomatic brinkmanship as the Trump administration grows frustrated by a lack of progress in its efforts to end the bloodiest conflict in Europe in generations.

On Monday, the Kremlin spokesman brushed off the threat.

“The language of ultimatums is unacceptable — you cannot talk to Russia like this,” the spokesman, Dmitri S. Peskov, told Russian news agencies.

President Vladimir V. Putin of Russia has so far rejected an unconditional 30-day truce that was first proposed by the United States in early March and immediately accepted by Ukraine.

Instead, Mr. Putin called this weekend for the resumption of direct negotiations with Ukraine. President Volodymyr Zelensky of Ukraine responded by challenging the Russian leader to meet him in person.

“I will be in Turkey this Thursday, May 15, and I expect Putin to come to Turkey as well. Personally,” Mr. Zelensky said on Sunday. “And I hope that this time, Putin won’t be looking for excuses as to why he ‘can’t’ make it.”

Mr. Peskov would not comment on Mr. Zelensky’s gambit.

Andriy Yermak, the chief of staff to Mr. Zelensky, told the Ukrainian news outlet RBC-Ukraine that Kyiv was accustomed to the Kremlin’s stalling tactics. The offer to meet with the Russian president, he said, was intended to make it clear that the “the ball is in Moscow’s court.”

“We won’t give Russia reason to accuse us of sabotaging the peace process,” Mr. Yermak added.

Ukraine, along with its European allies, continued to insist on Monday that the fighting needed to stop before serious negotiations could get underway.

President Emmanuel Macron of France said that Kyiv had shown itself willing time and again to make concessions in order to begin the hard work of brokering a lasting peace.

“Either Russia — Mr. Putin — is serious and wants peace, or it’s not serious and we have to impose even more sanctions,” he told reporters.

Britain’s foreign secretary, David Lammy, told reporters in London that it was time for Russia to stop stalling. “This is the time for Vladimir Putin to get serious about peace in Europe, to get serious about a cease-fire, and to get serious about talks,” he said.

When the leaders of France, Germany, Britain and Poland traveled to Kyiv over the weekend to deliver that same message, they were careful to coordinate their efforts with the White House. They made clear that only agreeing to an unconditional cease-fire would spare Moscow additional sanctions targeting oil exports and banking — and said that the United States would also impose sanctions on Russia if Mr. Putin did not agree to a truce.

However, after Mr. Putin made a counteroffer of direct talks without mentioning a cease-fire, President Trump urged the Ukrainians to take the meeting.

“Ukraine should agree to this, IMMEDIATELY,” Mr. Trump wrote on Sunday in a statement on social media.

Mr. Zelensky responded swiftly, seemingly seeking to flip the script on the Russian leader by saying he agreed to negotiations in any format and that he would travel to Turkey to meet with Mr. Putin — personally.

Nataliya Vasilyeva, Christopher F. Schuetze, Aurelien Breeden and Stephen Castle contributed reporting.