Washington DC - President Donald Trump on Monday came out with some rare public criticism of Russia as it continues bombing Ukraine despite his administration's push for a pause in the fighting.

"I'm not happy about what's going on" in Ukraine, the Republican told reporters in the White House, accusing Russia of "bombing like crazy right now" even as he said the parties were "sort of close" on a deal.

Moscow and Kyiv have been locked in war for the last three years, and Trump described the relentless attacks as "not a good situation."

"So we're meeting with Russia, we're meeting with Ukraine, and we're getting sort of close, but I'm not happy with all the bombing that's going in the last week or so," he said. "It's a horrible thing."

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The US leader said in March that he was "pissed off" with Russian President Vladimir Putin, and on Sunday told reporters: "We'd like them to stop. I don't like the bombing."

Trump's Monday remarks came hours after the Kremlin said it supported the idea of a truce in Ukraine but had many "questions" about how such a deal would work, pushing back at US and European suggestions that it was playing for time.

Russia has kept up its strikes on Ukraine despite the US president's promise to bring peace within "24 hours" of returning to the White House in January.

Putin rejected a joint US-Ukrainian proposal for an unconditional and full ceasefire in March.