President Trump on Thursday offered reassurances that Ukraine would be involved in negotiations to end the war with Russia, a day after his remarks left that prospect in doubt and alarmed officials in Kyiv and Ukraine’s European allies that they would be left out of peace talks.

“Of course they would,” Mr. Trump said in response to a reporter’s question in the Oval Office in Washington about whether Ukraine would have a place at the table. “I mean, they’re part of it. We would have Ukraine, we would have Russia, and we would have other people involved, too.”

European leaders on Thursday demanded a role in any discussions between Mr. Trump and President Vladimir V. Putin of Russia to end the conflict, negotiations that could set the terms of peace and redraw the map of Europe.

Some of the closest allies of the United States, including Britain and Germany, were among those asserting that Ukraine — and their own countries — had a right to be at the negotiating table.

“Europe must be involved in the negotiations — and I think that’s very easy to understand,” said Boris Pistorius, Germany’s defense minister. The continent, he said, “will have to live directly” with the consequences of any deal, and may have “to play a central or the main role in the peace order.”