US President Donald Trump said he is considering more sanctions on Moscow after Russia launched 367 drones and missiles at Ukrainian cities overnight, including the capital Kyiv.
The strikes killed at least 12 people, including three children in the northern region of Zhytomyr, Ukrainian officials said.
"I'm not happy with what Putin's doing. He's killing a lot of people," Trump told reporters in New Jersey on Sunday just before boarding his plane for a return to the White House from his Bedminister golf club.
Upon returning to Washington, Trump posted on social media that Putin had "gone absolutely CRAZY!"
The Russian attack was the largest of the war in terms of weapons fired, although other strikes have killed more people.
Russia denied targeting civilians in what the Kremlin calls its "special military operation" in Ukraine.
The latest assault comes as Ukraine and Russia conducted the third and final day of a prisoner swap in which both sides exchanged a total of 1000 people each.
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