Rishi Sunak and Volodymyr Zelensky have held talks on ramping up military support as Vladimir Putin's troops launch a bloody new offensive.
The PM told the Ukrainian president that Britain and its allies "needed to continue to ensure Ukraine was in the strongest possible position to build on its recent battlefield successes".
They spoke this morning as the Ministry of Defence warned Ukrainian soldiers were being bombarded with "particularly intense" Russian artillery fire in the highly-contested city of Bakhmut.
According to British intelligence reports, Ukrainian forces have been forced to make "orderly withdrawals" from positions they previously held there.
The call comes after gruesome footage emerged apparently showing a Ukrainian soldier being beheaded by a Russian serviceman.
A Downing Street spokeswoman said: "The leaders discussed the latest situation on the battlefield and the Prime Minister paid tribute to the efforts of the Ukrainian forces in Bakhmut.
"Discussing the abhorrent beheading of a Ukrainian soldier shown on social media in recent days, the Prime Minister said the video was appalling and those responsible had to be held to account.
"The leaders also discussed efforts to accelerate military support to Ukraine, and the Prime Minister said the UK and its allies needed to continue to ensure Ukraine was in the strongest possible position to build on its recent battlefield successes.
"That included increasing interoperability with Nato both in the short and long term, the Prime Minister added.
"The leaders agreed to stay in close touch."
President Zelensky came to the UK for his first visit since the Russian invasion of his country in February.
On his surprise trip, he delivered an historic address in Westminster Hall where he hailed the bravery of Britain in backing Ukraine against Russian aggression.
The heroic war leader vowed that "freedom will win" and said victory over Russia would "change the world" in his first visit to London since Vladimir Putin's brutal invasion began in February last year.
The visit was only President Zelensky's second outside Ukraine since the war began after he made a whirlwind trip to the United States in December.
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