US President Donald Trump has said he wanted Russia's Vladimir Putin to "stop shooting" in Ukraine and sign a peace deal, one day after the US leader met Ukraine's president at the Vatican.

Trump, who boasted before his inauguration that he could halt the Russia-Ukraine war within one day, has launched a diplomatic offensive since taking office to halt the fighting.

Those efforts have so far failed to yield any results.

"Well, I want him to stop shooting, sit down, and sign a deal," Trump said in response to a question on what he wanted from Putin on Sunday.

Trump was speaking on the tarmac at Morristown airport before boarding Air Force One bound for Washington, having attended the funeral of Pope Francis in Rome on Saturday.

"We have the confines of a deal, I believe, and I want him to sign it," Trump added, likely referring to a US-proposed peace plan for the more-than-three-year-long conflict in Ukraine.

Trump had met with Ukrainian President Volodymyr Zelenskyy at the funeral, where the two leaders spoke face-to-face for the first time since a disastrous televised meeting in the White House in February.

After their brief talk in St Peter's Basilica, Trump cast doubt over whether Putin wanted an end to the war, which has devastated swaths of eastern Ukraine and killed tens of thousands of people.

Trump also said on Sunday that he thought Zelenskyy was ready to give up Crimea, the Black Sea peninsula annexed by Russia in 2014, as part of efforts to agree on a peace deal.

"Oh, I think so," said Trump in response to a question on whether he thought Zelenskyy was ready to "give up" the territory.