British Prime Minister Keir Starmer met Donald Trump Thursday to plead for a US "backstop" to any Ukraine ceasefire, insisting it would be the only way to deter Russia's Vladimir Putin from invading again.
In their Oval Office meeting, Starmer will push Trump for US aerial surveillance and even air power to support peacekeepers that Britain and France are offering to deploy if the conflict ends.
But Trump has been wary of committing American resources, adding to concerns in Europe that the US president will abandon Kyiv and take Russia's side in negotiations.
Starmer's visit comes days after a similar visit by French President Emmanuel Macron, who came away effectively empty-handed despite saying there had been a "turning point" with Trump.
"The security guarantee has to be sufficient to deter Putin," Starmer told reporters on the plane to Washington.
"If there is a ceasefire without a backstop, it will simply give him the opportunity to wait and to come again, because his ambition in relation to Ukraine is pretty obvious."
British officials said European countries would be reluctant to commit forces unless Washington had their backs.
'Trade-off'
But Trump has long pushed for European nations to take more of the burden for Ukraine's defence, and their own.
"I'm not going to make security guarantees beyond very much. We're going to have Europe do that," Trump told reporters as he held the first cabinet meeting of his second term on Wednesday.
A senior Trump administration official said the backstop was "obviously very high on our European allies' agenda" but said securing a proper ceasefire first was more important.
"The type of force depends very much on the political settlement that is made to end the war. And I think that trade-off is part of what the leaders today are going to be discussing," the official told reporters.
The meeting promises to be a clash of styles between the mild-mannered Labour leader, a former human rights lawyer, and the brash Republican tycoon.
Starmer, who will hold a joint press conference with the US president, has pitched himself as a "bridge" between Trump and Europe on Ukraine.
The British premier also comes bearing a gift for Trump.
His announcement Tuesday that UK defense spending will rise to 2.5 percent by 2027 was particularly aimed at the American leader, who has badgered European countries to pay more towards NATO.
"We are very pleased with Prime Minister Starmer’s announcement," the senior US official said.
'Nice guy'
Starmer will meanwhile hope to avoid sweeping tariffs that Trump has promised to slap on the European Union.
Trade "will certainly be part of the conversation," added the US official.
But like Macron on Monday, he will have his work cut out to persuade Trump on Ukraine.
Last week Trump called Starmer a "very nice guy" - but complained that he and Macron had done "nothing" to end the war in Ukraine.
The US president stunned allies when he began negotiations with Russia, without including Ukraine or its European allies.
Concerns deepened when Trump attacked Ukrainian President Volodymyr Zelensky as a "dictator" and echoed Moscow talking points blaming Kyiv for Russia's February 2022 invasion.
Despite that, there have been growing signs of movement on a deal to end more than three years of bloody fighting.
Zelensky is due at the White House on Friday to sign a deal giving Washington access to Ukraine's rare minerals, which Trump has demanded as payback for US military aid.
The Ukrainian president is hoping the deal will provide a guarantee of future US support.
Starmer is then hosting Zelensky, Macron and other European leaders in Britain on Sunday as momentum grows.
In Moscow, Putin on Thursday said that initial Russia-US talks "give some hope" of resolving "problems" like the Ukraine conflict.