Heading to Washington for a meeting with U.S. President Donald Trump, Starmer again said Britain would "play our part" in any peacekeeping role in Ukraine, but could only do so with some kind of backup from the United States.

Since Trump abandoned Washington's more Ukraine-friendly approach to Russia's war, Britain and other European nations have scrambled to demonstrate a united front and stepped up diplomatic efforts in support of Kyiv.

But there are still differences over any future deployment of troops in Ukraine, with some European nations including France ready to do so, while others, such as Poland, have ruled it out.

Russia again on Wednesday said it could not consider "any options" for European peacekeepers being sent to Ukraine.

For Starmer, the crucial part of plans for a Ukraine peace deal is securing what he calls a "backstop" from Trump, something that has yet to be clearly defined.

"I'm absolutely convinced that we need a lasting peace, not a ceasefire, and for that to happen we need security guarantees," he told reporters travelling with him to Washington.

"Precisely what that layers up to, what that looks like, is obviously a subject of intense discussion."

Starmer said the backstop was crucial in order to deter Russian President Vladimir Putin from attacking Ukraine again.

"... my concern is if there is a ceasefire without a backstop, it will simply give him (Putin) the opportunity to wait and to come again because his ambition in relation to Ukraine is pretty obvious, I think, for all to see," he said.

A day after saying he would boost defence spending - meeting a demand often made by Trump to members of the NATO alliance - Starmer was reticent about his chances of success in Washington.

"I'm not going to get ahead of myself on it, other than to say I'm very clear about what the principles are," he said.

Before Starmer's arrival, Trump said: "I'm not going to make security guarantees beyond very much. We're going to have Europe do that, because we're talking about Europe as the next door neighbour (of Ukraine)."

STARMER FOLLOWS MACRON

Starmer's trip comes on the heels of a good-humoured visit to Washington by French President Emmanuel Macron who agreed with Trump on the deployment of European peacekeeping forces. However Macron also did not get a firm nod on the backstop idea.

The prime minister and his team are hoping to press on with a charm offensive begun at a two-hour dinner they shared with Trump last September at Trump Tower, a meeting British officials said was warm, with the "gracious host" offering foreign minister David Lammy a second helping of chicken.

Starmer said he wanted the so-called "special relationship" between the U.S. and Britain - a term first referenced by Winston Churchill after World War Two - "to go from strength to strength".

But he will have to navigate some tricky differences.

Trump blindsided Europe by calling Putin without warning and sending a delegation to Saudi Arabia for talks with Russia without including Ukraine or Europe. Trump has called Ukrainian President Volodymyr Zelenskiy a "dictator" and falsely suggested that Kyiv was responsible for starting the war.

Starmer has been careful to stick to his positions - that Putin started the war with the 2022 full-scale invasion, that Zelenskiy is democratically elected and Ukraine should be involved in peace negotiations - without addressing Trump's comments directly.

He said there was "one aggressor here and that's Russia" and was immediately asked whether Trump agreed with his thinking.

"Of course, the president has been very clear about the peace that he wants. He's right about that. We all want peace," he said. "The question is, how do we make sure it's a lasting peace. There's no issue between us on this."