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.