US President Donald Trump called his Russian counterpart Vladimir Putin "crazy" on Sunday 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. He has gone absolutely CRAZY!" Trump said in a post on his Truth Social platform.

"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!" he added.

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