EPAUS President Donald Trump's actions have brought the end of the war in Ukraine "closer than ever before", Prime Minister Sir Keir Starmer has said.
His statement comes despite the fact that Trump and Russian President Vladimir Putin failed to reach a deal on a ceasefire at a summit in Alaska on Friday.
He also said the"path to peace in Ukraine" cannot be decided without Ukrainian President Volodymyr Zelensky, and that the UK's"unwavering support" will continue as long as it takes.
The highly-anticipated meeting between Trump and Putin had been pitched as an important step towards peace in Ukraine, but no concrete agreements were made despite the former saying"great progress" was made.
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Zelensky is due to fly to Washington DC on Monday to meet Trump, with the aim of paving the way to further talks.
In the wake of the Anchorage summit, Sir Keir spent Saturday morning speaking to Western allies.
Following the calls, he said in a statement:"I welcome the openness of the United States, alongside Europe, to provide robust security guarantees to Ukraine as part of any deal.
"President Trump's efforts have brought us closer than ever before to ending Russia's illegal war in Ukraine.
"His leadership in pursuit of an end to the killing should be commended," Sir Keir said.
Until Putin stops his"barbaric assault", allies would"keep tightening the screws on his war machine with even more sanctions", he added.
France later said Sir Keir would join French President Emmanuel Macron and German Chancellor Friedrich Merz in hosting Kyiv's allies in a video call of the"coalition of the willing" on Sunday.
A Downing Street source told the BBC that any peace deal needed security agreements and"US involvement is a key part of that".
Following a call with Trump on Saturday, Zelensky called for a lasting peace,"not just another pause between Russian invasions".
He stressed Kyiv should be included in future discussions, and said he expected Russia to"increase pressure and strikes" in the coming days to "create more favourable circumstances for talks with global actors".
Prior to the event, it was believed Trump had hoped to secure a peace deal from the talks in Alaska. Speaking to Fox News Radio on Thursday, Trump said the meeting had only a 25% chance of failure.
Despite walking away with no ceasefire and no overall deal, Trump insisted"some great progress" was made, with"many points" agreed and "very few" remaining.
On Friday, Zelensky visited Sir Keir at Downing Street, and the pair greeted each other in a warm embrace before holding talks over breakfast.
It was seen as a carefully co-ordinated show of support from the UK, ahead of the the Trump-Putin summit.
