US President Donald Trump said Thursday he was applying pressure on Russia to end the Ukraine war, insisting that Moscow agreeing not to take over the entire country was a "big concession."

Trump issued a rare rebuke of Russian leader Vladimir Putin after Moscow fired a barrage of missiles and drones at Kyiv, killing at least 12 in the deadliest attack on the Ukrainian capital in months.

"We're putting a lot of pressure on Russia, and Russia knows that," said Trump, who has been accused of favoring Russia in the push to halt the war.

Asked what concessions Russia had offered, Trump said "stopping the war, stopping taking the whole country. Pretty big concession."

In response to the attack on Kyiv, he said: "I didn't like last night. I wasn't happy with it. And we're in the midst of talking peace, and missiles were fired."