The United Kingdom's attempt to help broker a peaceful end to the Russia-Ukraine conflict hit an embarrassing roadblock this week, when longtime ally the United States withdrew high-level support for the negotiations at the last moment.

Planned talks in London on Wednesday were supposed to have involved foreign ministers from several European countries as well as US Secretary of State Marco Rubio and US special envoy Steve Witkoff, but both US representatives pulled out hours before the talks were due to start.

The withdrawals led to a downgrading of the talks, from a gathering of senior politicians to one involving bureaucrats from France, Germany, the UK, Ukraine, and the US.

UK Foreign Secretary David Lammy took the opportunity to meet with Ukrainian counterpart Andrii Sybiha, but the failure of the talks and the US branding them "technical meetings" will have been disappointing for London.

The UK Foreign Office confirmed early on Wednesday the talks had been downgraded and would no longer involve foreign ministers, saying: "Official-level talks will continue but these are closed to media."

The BBC said Rubio spoke to Lammy on Tuesday night and told him he hoped for success in what he called the latest in a series of "substantive and good technical meetings".

Rubio reportedly told Lammy he will visit the UK at some time in the coming months.

The US secretary of state added on the social media platform X: "I look forward to following up after the ongoing discussions."

Keith Kellogg, Washington's Ukraine envoy, attended Wednesday's talks while Rubio and Witkoff focused on planned talks in Moscow later this week that will reportedly involve a fourth meeting between Witkoff and Russia's President Vladimir Putin.

The downgrading of Wednesday's talks followed US President Donald Trump warning on Sunday that Washington could walk away from peace talks completely, if they fail to achieve a breakthrough soon. He added that he wants a peace deal in place by the end of April.

Andriy Yermak, an advisor to Ukraine's government, said as he arrived in London ahead of the downgraded talks that officials planned to focus on ways to achieve an unconditional ceasefire, and would defer discussion of conditions either side might have on a lasting post-conflict peace deal.

"Despite everything, we will work for peace," he wrote on social media.

The London gathering followed one in Paris last week that involved US, Ukrainian, and European officials.

Witkoff has previously said both Ukraine and Russia will need to make concessions if they are to achieve a lasting peace, the Reuters news agency said, citing unnamed diplomats.

France's Foreign Minister Jean-Noel Barrot said on Tuesday that European nations will not, however, agree to recognize Russia's sovereignty over Crimea and will insist it is returned to Kyiv's control.