US President Donald Trump was unhappy but not surprised by Russia's strikes on the Ukrainian capital Kyiv, the White House said Thursday -- while urging "both sides" to end the war started by Moscow's invasion.

"He was not happy about this news, but he was also not surprised. These are two countries that have been at war for a very long time," Press Secretary Karoline Leavitt told a briefing.

"Russia launched this attack on Kyiv, and likewise, Ukraine recently dealt a blow to Russia's oil refineries."

Trump may make further comments on the issue later, Leavitt added.

At least 19 people were killed when Russian missiles and drones ripped through apartment blocks in one of the biggest attacks of the war. The European Union mission and a British government cultural building were also hit.

Trump promised to end the Ukraine war within 24 hours of taking office but has since admitted it is tougher than he expected.

A summit with Russian President Vladimir Putin in Alaska earlier this month failed to produce a breakthrough.

Leavitt said Trump had worked "harder than anyone" for peace but echoed earlier comments by Trump that have also sought to put some blame on Ukraine, which was invaded by Russia in February 2022.

"Perhaps both sides of this war are not ready to end it themselves," Leavitt said.

"The president wants it to end, but the leaders of these two countries... must want it to end as well." 

Trump's special envoy for Ukraine, Keith Kellogg, had earlier warned that the Russian strikes on "innocent civilians" could undermine the president's peace efforts.

"These egregious attacks threaten the peace that @POTUS is pursuing," Keith Kellogg said on X.