Sir Keir Starmer spoke with US President Donald Trump and French President Emmanuel Macron after welcoming Volodymyr Zelensky to Downing Street.
The calls took place after the Prime Minister insisted Ukraine had “full backing across the United Kingdom” in a No 10 meeting with the country’s president, the PA news agency understands.
Sir Keir also told the Ukrainian leader that Britain stands with his nation “for as long as it may take”.
The Prime Minister’s call to Mr Trump comes after the US president’s unprecedented public clash with Mr Zelensky in the White House’s Oval Office.
The spat has shaken the foundations of Nato, with European allies rallying to Ukraine’s side, while Mr Trump’s American backers have insisted he was standing up for their country’s interests.