MORRISTOWN, United States, May 26, 2025 (BSS/AFP) - US President Donald Trump issued a rare rebuke to Russian leader Vladimir Putin on Sunday, saying he was "not happy" with him after Moscow launched a record number of drones against Ukraine, killing 13 people across the country.

Trump has expressed admiration for Putin in the past, but has in recent weeks shown growing frustration with Moscow's position in truce negotiations with Kyiv, which are deadlocked.

"I'm not happy with what Putin is doing. He's killing a lot of people, and I don't know what the hell happened to Putin," said Trump on the tarmac at Morristown airport before boarding Air Force One bound for Washington.

"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."

Russia's attacks on Ukraine came as the two countries completed their biggest prisoner swap since Moscow launched its full-scale invasion in February 2022, with 1,000 captured soldiers and civilian prisoners sent back by each side.

Those killed in the latest Russian strikes included two children, aged eight and 12, and a 17-year-old, killed in the northwestern region of Zhytomyr, officials said.

"Without truly strong pressure on the Russian leadership, this brutality cannot be stopped," Ukrainian President Volodymyr Zelensky said on social media.

Responding to a question on the tarmac in Morristown, 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," said the US leader.

That statement was at odds with US Secretary of State Marco Rubio's testimony at Congress earlier this 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 on Monday 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.