After their summit in Alaska, President Trump sided on Saturday with President Vladimir V. Putin of Russia, adopting Mr. Putin’s preference for pursuing a sweeping peace agreement based on Ukraine’s ceding unconquered territory to Russia instead of the urgent cease-fire Mr. Trump had said he wanted before the meeting.

The change could give Russia an advantage in talks to end the fighting, which are due to continue on Monday when President Volodymyr Zelensky of Ukraine visits the White House. It would also be a break from the strategy that Mr. Trump and European allies, as well as Mr. Zelensky, had agreed to before the U.S.-Russia summit in Alaska, and it provoked a chilly reception in Europe, where leaders have time and again seen Mr. Trump reverse positions on Ukraine after speaking with Mr. Putin.

Mr. Trump wrote on social media early on Saturday that he had spoken by phone to Mr. Zelensky and other European leaders after his meeting with Mr. Putin. “It was determined by all that the best way to end the horrific war between Russia and Ukraine is to go directly to a Peace Agreement, which would end the war, and not a mere Ceasefire Agreement, which often times do not hold up,” Mr. Trump posted.

Mr. Trump told European leaders that he believed a rapid peace deal could be negotiated if Mr. Zelensky agreed to cede the rest of the eastern Donbas region to Russia, even those areas not occupied by Russian troops, according to two senior European officials briefed on the call.

In return, Mr. Putin offered a cease-fire in the rest of Ukraine at current battle lines and a written promise not to attack Ukraine or any European country again, the senior officials said. Mr. Putin has broken similar promises before. The officials spoke on condition of anonymity to discuss a private conversation.