President Trump has said Russia’s deadly missile attack on the Ukrainian city of Sumy was “a mistake,” calling it a “horrible thing” even as members of his administration went further with condemnation that served as a rare critique of Moscow while the White House is pushing for a cease-fire.

Two ballistic missiles hit the center of Sumy on Sunday morning, killing at least 34 people and injuring more than 100. The attack was the second in just over a week to cause large numbers of civilian casualties in Ukraine, which Kyiv has said shows that the Moscow is not truly interested in stopping the fighting despite U.S.-led negotiations for a truce.

“I think it was terrible. And I was told they made a mistake. But I think it’s a horrible thing,” Mr. Trump said about the Sumy attack when he was aboard Air Force One on Sunday.

It was not immediately clear what Mr. Trump meant when he said he had been “told” Russia had “made a mistake” — a formulation that could also be interpreted as an attempt to make excuses for Moscow. Mr. Trump has generally avoided criticizing President Vladimir V. Putin of Russia since taking office in January.

Some top officials in the Trump administration were more explicit. Keith Kellogg, a retired U.S. general and Mr. Trump’s special envoy for Ukraine, said that Russian forces had crossed “any line of decency” by targeting civilians in Sumy. Secretary of State Marco Rubio also condemned what he described as a “horrifying Russian missile attack on Sumy.”

“This is a tragic reminder of why President Trump and his Administration are putting so much time and effort into trying to end this war and achieve durable peace,” Mr. Rubio wrote on social media.