US President Donald Trump and Ukrainian President Volodymyr Zelensky have clashed again on efforts to end the three-year-old war in Ukraine, with the US leader chiding Zelensky for refusing to recognise Russia's occupation of Crimea.

Trump's Vice President JD Vance said it was time for Russia and Ukraine to either agree to a US peace proposal "or for the United States to walk away from the process," echoing a warning Trump gave last week.

Speaking to reporters in India, Vance said the proposal called for freezing territorial lines "at some level close to where they are today" and a "long-term diplomatic settlement that hopefully will lead to long-term peace."

"The only way to really stop the killing is for the armies to both put down their weapons, to freeze this thing," he said.

A former Western official familiar with the US proposal said it also called for the recognition of Russia's annexation of Crimea.

Since taking office in January, Trump has upended US policy toward the war in Ukraine, pressing Ukraine to agree to a ceasefire while easing pressure on Russia, which launched a full-scale invasion of its neighbour in 2022.

Zelensky on Tuesday reiterated that Ukraine would never cede Crimea to Russia. "There’s nothing to talk about here. This is against our constitution."

Trump, who argued with Zelensky in a televised Oval Office meeting in March, called this an inflammatory statement that made a peace deal harder to achieve.

He said Crimea was lost years ago "and is not even a point of discussion."

"If he wants Crimea, why didn’t they fight for it 11 years ago when it was handed over to Russia without a shot being fired?" Trump wrote on Truth Social.

Zelensky acknowledged later in a post on X that talks in London on Wednesday between US, Ukrainian and European officials had been marked by high emotions but expressed hope that future joint work would lead to peace.

He pledged again that Ukraine would abide by its constitution and said he was sure Kyiv's partners, in particular the United States, "will act in line with its strong decisions".

He attached to his post a 2018 Crimea Declaration from Mike Pompeo, Trump's secretary of state during his first term, which said: "the United States rejects Russia’s attempted annexation of Crimea and pledges to maintain this policy until Ukraine’s territorial integrity is restored."

Trump told reporters later he thought the London talks had gone "pretty well," although he also said, in apparent reference to Russian President Vladimir Putin and Zelensky:

"We've got to get two people, two strong people, two smart people, to agree. And as soon as they agree, the killing will stop."

"I thought it might be easier to deal with Zelensky. So far, it's been harder.

"But, I think we have a deal with both. I hope they do it, because I'm looking to save and, you know, we spent a lot of money, but this is about a lot of humanity," Trump said.

US Treasury Secretary Scott Bessent later met Ukrainian Prime Minister Denys Shmyhal and Finance Minister Sergii Marchenko and stressed the need to sign an economic partnership between the two countries as soon as possible, the US Treasury Department said.

US Secretary of State Marco Rubio cancelled his trip to attend the London talks prompting cancellation of a broader meeting with foreign ministers from Ukraine, Britain, France and Germany, underscoring the gaps between Washington, Kyiv and its European allies over how to end the war.

Trump, who promised during his election campaign to end the war within his first 24 hours back in the White House, scolded Zelensky and said the US was trying to stop the killing in Ukraine and that they were "very close to a deal" for peace.

White House spokeswoman Karoline Leavitt said Trump is "frustrated" with the pace of talks and that Zelensky "seems to be moving in the wrong direction".

White House says Zelensky moving in wrong direction on peace talks. – Reuters

Russian fighters seized control of the Crimean peninsula in 2014 in a move that was condemned internationally. Few countries recognise Russia's claim to Crimea.

Several sources have said proposals from Trump's envoy Steve Witkoff include not only recognising Russia's annexation of Crimea, but accepting Russia's control of the 20 per cent of Ukraine's territory it has gained in the war, ruling out Ukrainian membership of NATO, and lifting of Western sanctions.

Trump's Ukraine envoy Keith Kellogg said on X that there were positive talks in London with Zelensky's chief of staff, Andriy Yermak, and added:

"It's time to move forward on President Trump's UKR-RU war directive: stop the killing, achieve peace, and put America First."

Trump raised the pressure on Sunday when he said he hoped Moscow and Kyiv would make a deal this week to end the conflict.

At the heart of Wednesday's talks was an attempt to establish what Kyiv could possibly accept after Witkoff presented proposals to a similar session in Paris last week.

Three diplomats said those proposals appeared to demand more concessions from Ukraine than Russia.

• RUSSIA: Putin willing to discuss truce proposal

Witkoff is expected to meet Russian President Vladimir Putin again on Friday, a US official said.

Witkoff has already met Putin three times to discuss prospects for an end to the war and will visit Moscow this week for a new round of talks, the White House said earlier.

Since Trump expressed his desire to broker peace in Ukraine and made a surprise call to Putin in February, European nations have scrambled to find ways to support Kyiv against Moscow while keeping the US onside.

A joint statement from Britain, France and Germany after the London talks said all parties had reiterated strong support for Trump's "commitment to stopping the killing and achieving a just and lasting peace."

It said "significant progress was made on reaching a common position on next steps" and "all agreed to continue their close coordination and looked forward to further talks soon."

Ukraine peace talks 'heading back into conflict,' analyst says. – Reuters