U.S. 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.