Donald Trump's meeting in the Vatican with Ukrainian leader Volodymyr Zelensky was hailed by both sides as one for the history books.

Whether it will help seal a ceasefire deal between Ukraine and Russia will become clearer in the coming days.

Brought together by the funeral of Pope Francis, the US and Ukrainian presidents sat knee-to-knee on red and gold chairs in the giant nave of Saint Peter's Basilica, in their first meeting since their blazing televised fight in the Oval Office in February.

"How iconic," Trump's National Security Advisor Mike Waltz told Fox News's Sunday Morning Futures in the first detailed comments from the White House since Saturday's encounter.

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"That setting, that scene, that meeting is going to go down in the history books. For President Trump to be a president of peace, to be talking peace and diplomacy in the Vatican, of all places."

Zelensky gave a similar verdict, saying it was a "very symbolic meeting that has potential to become historic, if we achieve joint results."

And whatever Zelensky said during the Vatican encounter, it appeared to work.

Hours afterwards, Trump appeared to turn against Vladimir Putin for the first time – a sharp reversal given his recent favorable tone towards the Russian leader.

Trump said on Truth Social that Russia's continued attacks "makes me think maybe he doesn't want to stop the war, he's just tapping me along" and raised the possibility of fresh sanctions on Moscow.

But Trump has ultimately shown increasing frustration and impatience with both sides, as he tries to secure a peace deal he once bragged he could seal within 24 hours.

And while the Zelensky meeting may have bought some time, the White House reiterated its stance that without rapid progress, it will soon walk away from its role as a broker.

US Secretary of State Marco Rubio said Sunday that "this is going to be a very critical week" for efforts to end the war.

"We're close, but we're not close enough," Rubio told NBC's Meet the Press.

Waltz said that in the Vatican meeting, Trump wanted to "talk to Zelensky face-to-face and talk about how we're going to get the largest land war in Europe to an end."

"Both sides have to want that. He's expressed his frustration with both sides, but yet his determination," said the US national security advisor, who was also at the pope's funeral.