British Prime Minister Keir Starmer on Saturday gave Volodymyr Zelensky a warm welcome in London, a day after the Ukrainian leader's clash with US President Donald Trump.

And ahead of Sunday's summit in London to discuss how to support Ukraine as it battles Russia's forces, Starmer reiterated his support for Kyiv.

"In partnership with our allies, we must intensify our preparations for the European element of security guarantees alongside continued discussions with the United States," he said in a statement issued late Saturday.

"Now is the time for us to unite in order to guarantee the best outcome for Ukraine, protect European security, and secure our collective future," he added.

Hours earlier, Ukraine and the UK unveiled a £2.26 billion (US$2.84 billion) loan agreement to support Ukraine's defence capabilities, to be paid back with the profits of immobilised sovereign Russian assets.

"The funds will be directed toward weapons production in Ukraine," Zelensky said on X.

"This is true justice -- the one who started the war must be the one to pay."

Supporters cheered as Zelensky's convoy swept into Downing Street, where he was embraced by Starmer and posed for photographs before heading inside the British leader's home.

"You're very, very welcome here in Downing Street," Starmer told Zelensky.

"I want to thank you, the people of the United Kingdom, for such big support from the very beginning of this war," Zelensky responded.

He is due to meet King Charles III on Sunday.

The leaders met behind closed doors for around 75 minutes, and embraced again as Starmer escorted Zelensky to his car.

Earlier Saturday Zelensky had stressed that Trump's support was still "crucial" for Ukraine despite their row the previous day.

The clash was a further shock to Kyiv's European allies, still adjusting to Washington's new stance on the war.

On Friday as cameras in the Oval Office rolled, Trump berated Zelensky for not being "ready" for peace with Russia, triggering alarm across Europe. (AFP)