US President Donald Trump has called his Russian counterpart Vladimir Putin "crazy" after Moscow launched a deadly barrage of drones against Ukraine, even as the warring countries completed a large-scale prisoner exchange.

At least 13 people were killed when Russia launched a record number of drones against Ukraine overnight to Sunday.

"I've always had a very good relationship with Vladimir Putin of Russia, but something has happened to him," Trump said in a post on his Truth Social platform on Sunday.

"He has gone absolutely CRAZY!

"I've always said that he wants ALL of Ukraine, not just a piece of it, and maybe that's proving to be right, but if he does, it will lead to the downfall of Russia!"

His comments marked a rare rebuke to Putin, who he often speaks of with admiration.

The US leader has, however, expressed increasing frustration with Moscow's position in deadlocked truce negotiations with Kyiv.

Earlier on Sunday, Trump said he was "not happy" about the latest attack on Ukraine and that he was "absolutely" considering increasing sanctions on Moscow.

"I've known him a long time, always gotten along with him, but he's sending rockets into cities and killing people, and I don't like it at all," he said. (AFP)