President Trump said he would speak with President Vladimir V. Putin of Russia on Monday to try to secure a cease-fire in the war between Russia and Ukraine.

“HOPEFULLY IT WILL BE A PRODUCTIVE DAY, A CEASEFIRE WILL TAKE PLACE, AND THIS VERY VIOLENT WAR, A WAR THAT SHOULD HAVE NEVER HAPPENED, WILL END,” Mr. Trump wrote, in all caps, on social media on Saturday. The Kremlin confirmed that the leaders would speak.

Mr. Trump said he would also call President Volodymyr Zelensky of Ukraine, who has agreed to an unconditional and immediate cease-fire, as well as leaders of the NATO alliance.

Friedrich Merz, Germany’s new chancellor, told reporters in Rome on Sunday that he had spoken with Secretary of State Marco Rubio about Mr. Trump’s upcoming phone call with Mr. Putin. Speaking with reporters after attending Pope Leo XIV’s inaugural Mass, Mr. Merz said that he and the leaders of France and Britain were planning to try to speak with Mr. Trump before that call.

“My strong impression is that both Europeans and Americans are determined to work together, but now also in a focused manner, to ensure that this terrible war ends soon,” Mr. Merz said.

Their conversations will take place as Russia intensifies its attacks in Ukraine, including a drone strike on Saturday against a civilian bus that killed at least nine people, according to local residents and Ukrainian authorities.