United States President Donald Trump called Russian counterpart Vladimir Putin “crazy” for his attacks on Ukrainian cities and warned that any attempt at a total takeover of Ukraine would “lead to the downfall of Russia”.

The comments were a rare rebuke to President Putin, and came after a record number of Russian drones killed at least 13 people across Ukraine, despite a prisoner exchange and a US push for a truce.

“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 Truth Social.

“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.

Trump also criticised Ukrainian President Volodymyr Zelenskyy, a more frequent target of his ire, accusing him of “doing his country no favours by talking the way he does”.

“Everything out of his mouth causes problems, I don’t like it, and it better stop,” he said of Zelenskyy.

Earlier, Trump told reporters that he was “not happy” with Putin over the latest Russian offensive.

“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,” said Trump on the tarmac at Morristown Airport before boarding Air Force One for Washington.

Trump’s remarks come as European allies and even some in his own Republican Party call for increased pressure on Russia to agree to a ceasefire.

The US President has avoided issuing ultimatums to Russia, instead threatening to walk away from negotiations if both sides cannot agree to a ceasefire.

But in response to a question, Trump said he was “absolutely” considering increasing US sanctions on Russia in response to the latest violence.

“He’s killing a lot of people. 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,” the US leader said.

That statement was at odds with US Secretary of State Marco Rubio’s testimony at Congress last week, when he said Trump believed that “right now, if you start threatening sanctions, the Russians will stop talking”.

Trump and Putin held a two-hour phone call last Tuesday NZT, after which the US leader said Moscow and Kyiv would “immediately start negotiations towards a ceasefire”.

Putin has made no commitment to pause his three-year invasion of Ukraine, announcing only a vague proposal to work on a “memorandum” outlining Moscow’s demands for peace.

-Agence France-Presse