Rishi Sunak and Volodymyr Zelensky discussed ramping up military support to Ukraine in a phone call on Friday, Downing Street said.
The Prime Minister 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”.
The two leaders discussed the latest developments in Russia’s invasion, with Mr Sunak paying tribute to the efforts of Ukrainian troops in the highly-contested town of Bakhmut.
Russia has “re-energised” its assault on Bakhmut with “improved co-operation” between troops fighting with Moscow’s defence ministry and the Wagner mercenary group, according to the MoD.
Mr Sunak also said a gruesome video that purportedly shows the beheading of a Ukrainian soldier was “appalling” and that those responsible had to be held to account.
Mr Zelensky has called on world leaders to respond to the video, which the Kremlin said was “horrible” but needed to be verified.
The Ukrainian president tweeted: “Had a phone talk with (UK) Prime Minister @RishiSunak. Discussed the situation at the front, our defense needs, increased support & planned intl events.
“I thanked him for condemning the inhumane execution of a (Ukrainian) soldier. Together we must stop the aggressor & put an end to terror!”
A Downing Street spokeswoman said: “The Prime Minister spoke to President Zelensky this morning.
“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.”
Mr Zelensky has lobbied western countries for months to supply modern warplanes, anti-aircraft defence systems and ammunition, while Ukrainian officials have spoken of preparations for an expected spring counter-offensive against Moscow’s troops.
Britain has offered to train Ukrainian pilots on Nato-standard jets but allies have been reluctant to release them.
Mr Sunak and Mr Zelensky’s call came after an alleged leak of US classified defence documents online appeared to lay bare military secrets in Ukraine, including claims that up to 50 UK special forces personnel have been deployed to the war-torn nation.
However, the MoD warned against taking allegations contained in the reported leak at “face value”.