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Sir Keir Starmer has said a Ukraine peace deal needs a "US backstop" to prevent Russia from attacking its neighbour again.
Speaking after a hastily called meeting with European leaders in Paris, the prime minister reiterated that he would consider deploying British troops to Ukraine if a lasting peace deal was reached.
He said "the only way to effectively deter Russia is through US security guarantees" and pledged to discuss the "key elements" of a peace deal with US President Donald Trump when the pair meet in Washington next week.
Chancellor Olaf Scholz of Germany, a key NATO ally, said it was "completely premature" to discuss sending troops to Ukraine at this time and that he was "a little uneasy" about the matter.
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Sir Kiir said Europe “must do more” to protect the continent in the face of Russia’s “generational” security challenges.
He was keen to avoid explaining exactly what he meant by “backstop” – but his allies think it could include air support, supplies and intelligence capabilities.
European leaders gathered at the Elysee Palace to discuss concerns over the Trump administration’s decision to start peace talks with Russia, which are due to begin in Saudi Arabia on Tuesday.
Ukrainian President Volodymyr Zelensky said Kiev was unaware of the talks and would not recognise any deal made without their involvement.
US officials have suggested that European countries would be consulted in peace talks with Russia, but would not be directly involved.
Early Tuesday morning, French President Emmanuel Macron tweeted that he had “just spoken” to Trump and Zelensky.
“We will work together with all Europeans, Americans and Ukrainians on this. That is the key,” he said, adding that he would “continue to discuss this in the coming days.”
The Paris summit comes days after US Defence Secretary Pete Hegseth said Europe would have to be primarily responsible for its own security in the future.
Sir Kiir said the US was “not leaving NATO” but “taking responsibility for our security, for our continent.”
Polish Prime Minister Donald Tusk said the transatlantic relationship was at a “new stage,” and that the meeting confirmed that it was time for “Europe to have more capacity to defend itself.”
Sir Kiir indicated that any troop contribution from the UK would be part of a multinational force to control the borders of Ukraine-occupied and Russian-occupied territories.
But experts say it would be a huge undertaking to do effectively, requiring a huge increase in defence spending.
Malcolm Chalmers, deputy director of the Royal United Services Institute, said that if the proposed force were to be intended to deter a Russian attack, contrary to the observation of the UN peacekeeping force, it would be a “completely different matter”.
“You need credible, well-equipped forces – and you need not only frontline forces, but also your back-up forces and air forces,” he told the BBC. “That is a huge demand.”
Former NATO commander General Sir Adrian Bradshaw said: “It cannot be a token force, it cannot be something that observes bad behaviour and stands by.”
He told BBC Radio 4's World at One that "NATO needs to do what it does on its own soil effectively - to actually deter aggression", and that it needs to be "influenced by a grand strategy for containing Russia" that makes it clear that any future conflict will not be confined to Ukraine.
"Basically, the force needs to be big enough to defeat an invasion," he added.
Former British Army chief Lord Dannatt previously estimated that such a force would require around 100,000 troops - with the UK contributing around two-fifths.
"We don't have that number yet," he said on Saturday, adding that the army would have to be well-equipped to play this role.