President Donald Trump on Sunday expressed deep unhappiness at Russia's weekend bombing of Ukraine, saying of Russian President Vladimir Putin, "I'm not happy with Putin."

"I don't know what's wrong with him. What the hell happened to him? Right? He's killing a lot of people. I'm not happy about that," Trump told reporters at the airport in Morristown, New Jersey, as he prepared to return to Washington.

Trump spoke in reaction to a Russian barrage of 367 drones and missiles at Ukrainian cities overnight on Sunday, including the capital Kyiv, in the largest aerial attack of the war so far, killing at least 12 people and injuring dozens more.

Trump has been trying to get both sides to agree to a ceasefire in the three-year-old war in Ukraine and he spoke for more than two hours with Putin last week.

He raised the possibility of imposing more sanctions on Russia in response to the ongoing attacks.

"Always gotten along with him, but he's sending rockets into cities and killing people, and I don't like it at all," Trump said.