US President Joe Biden has announced that he will end his candidacy for re-election, upending the race for the White House four months before Americans go to the polls in November.

The decision comes after weeks of intense pressure from fellow Democrats following a stumbling and sometimes incoherent debate performance against Republican Donald Trump at the end of June.

The 81-year-old had resisted calls to step aside even as concerns over his mental fitness and capacity to beat Trump mounted.

Following the decision to end his candidacy for re-election, President Biden also announced that he's endorsing Kamala Harris as the Democratic nominee for the US 2024 presidential race, in a post on X.

President Biden further announced that although he's dropping out of the presidential race, he's not stepping down as US president and will stay in the White House for the final six months of his term.

He said, "I believe it is in my best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as president for the remainder of my term".

Vice President Kamala Harris, issued a statement after learning that Joe Biden has ended his bid for re-election and endorsed her to become the Democratic candidate in the November presidential election.

She thanked President Biden for his extraordinary leadership and decades of service to the country.

She described his decision to end his re-election bid as a "selfless and patriotic act" and says she is "honored" to have Biden's endorsement.

President Joe Biden, 81, faced increasing calls to step aside since a faltering debate performance against Trump last month.