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.

Donald Trump and Volodymyr Zelensky met briefly in the hush of St Peter's basilica on the sidelines of Pope Francis's funeral on Saturday in their first encounter since a noisy White House clash, as the US president pushes the Ukrainian leader to make a peace deal with Russia.