Ukrainian President Volodymyr Zelensky on Thursday met with British Prime Minister Keir Starmer in London in a show of support on the eve of a key US-Russia summit from which Kyiv and its European allies have been excluded.

Starmer greeted the Ukrainian leader with a warm hug and handshake on the steps of his Downing Street residence, only hours after Zelensky took part in a virtual call with US President Donald Trump.

Trump and Russian President Vladimir Putin will meet on Friday at an air base in Alaska.

A stepped-up Russian offensive, and the fact Zelensky has not been invited to the Anchorage meeting, have heightened fears that Trump and Putin could strike a deal that forces painful concessions on Ukraine.

But Starmer said on Wednesday there was now a "viable" chance for a ceasefire in Ukraine after more than three years of fighting. (AFP)