Donald Trump complained that Volodymyr Zelenskyy has been harder to deal with than Russia as he lashed out at Ukraine's president on Wednesday, 23 April.
Mr Zelensky has ruled out ceding territory to Russia in any deal.
It comes as a high-level peace summit in London to discuss the terms of a Ukraine peace deal on Wednesday was abruptly downgraded after the US president's most senior diplomat snubbed the meeting.
Mr Trump, who is set to travel to Italy for Pope Francis's funeral on Saturday, told reporters that he did not know if he would meet with Zelensky or other European leaders to discuss the war while he is in Rome.