"The prime minister reiterated the UK's commitment to Ukraine being on an irreversible path to NATO, as agreed by allies at the Washington summit last year," a spokeswoman in Starmer's Downing Street office said.

It comes after US President Donald Trump's administration this week poured cold water on Ukraine's goal of eventually joining the Western military alliance.

The call also follows Trump and Russia's Vladimir Putin holding watershed talks Wednesday about Moscow's full-scale invasion, with the US leader announcing they had agreed to soon start Ukraine talks.

The developments have spooked Kyiv and its European allies, which fear a deal could be made over their heads, just as Zelensky and Western allies gathered Friday for the Munich Security Conference.

Starmer began his conversation with the Ukrainian leader by "reiterating the UK's concrete support for Ukraine, for as long as it's needed", according to the Downing Street spokeswoman's readout.

"He was unequivocal that there could be no talks about Ukraine, without Ukraine," she said.

"Ukraine needed strong security guarantees, further lethal aid and a sovereign future, and it could count on the UK to step up."

With the third anniversary of Russia's full-scale invasion looming, "the leaders agreed that it would be an important moment to demonstrate international unity and support for Ukraine," the spokeswoman added.