US President Donald Trump has followed up on a warning from his secretary of state about peace in Ukraine. He too suggests Washington could walk away from the negotiating table if either Kyiv or Moscow drag their feet.

Speaking to reporters at the White House on Friday, Trump said, "Now if, for some reason, one of the two parties makes it very difficult, we're just going to say, 'You're foolish, you're fools, you're horrible people,' and we're going to just take a pass."

But he also expressed his intention to continue dialogue, saying Washington wants to see the conflict end.

Secretary of State Marco Rubio said earlier in the day that if ending the fighting between Ukraine and Russia appears impossible, Trump will probably say, "Well, we're done."

Rubio also said, "We are not going to continue with this endeavor for weeks and months on end."

He later offered a similar view in phone talks with NATO Secretary General Mark Rutte, saying the United States could back off "if a clear path to peace does not emerge soon."