US President Joe Biden has ended his re-election campaign and endorsed Vice-President Kamala Harris to succeed him as the Democratic candidate, in an extraordinary decision that upends an already dramatic race for the White House.

Mr Biden, 81, said in a Sunday written statement that it was the "greatest honour" to serve but his withdrawal was "in the best interest of my party and the country".

The announcement caps a tumultuous period in US politics, which began with his sometimes incoherent debate performance against Donald Trump on 27 June. Mr Biden says he will remain president until January.

Ms Harris, 59, said that she was "honoured" to be endorsed, adding she would "earn and win this nomination" and unite the country against Trump.

"We have 107 days until election day," she said. "Together, we will fight. And together, we will win."

While Ms Harris has been picking up endorsements from many big figures in the party, she is yet to be officially nominated, and that may not happen until the Democratic National Convention in August.

A resurgent Trump meanwhile has pulled ahead in polling and was confirmed as Republican nominee at the party's convention in Milwaukee this week, five days after surviving an assassination attempt.

In the wake of Mr Biden's decision, he declared the president "was not fit to run... and is certainly not fit to serve". Other senior Republicans joined him in their criticism, and called on Mr Biden to leave the White House immediately, not just the Democratic candidacy.