US President Donald Trump and Ukrainian President Volodymyr Zelensky held crunch war talks in the heart of the Vatican minutes before the start of the funeral of Pope Francis on Saturday, as the White House mounts an increasingly urgent push to strike a peace deal in Ukraine.

The United States has been pushing Ukraine to accept a peace plan that seems in part a gift to Moscow. But the short meeting of the leaders, and subsequent comments, appeared to be a change in tone.

Trump on Saturday criticized Russia's Vladimir Putin for attacks on civilian areas of Ukraine in recent days, and said "maybe he doesn't want to stop the war."