United States President has pledged to continue his re-election campaign “to the end”, as the embattled Democrat fights to keep his candidacy alive amid growing alarm over his physical and mental fitness.

Joe Biden, 81, insisted that he would keep running despite growing pressure from within his party to step aside following last week’s disastrous debate performance against his Republican challenger Donald Trump.

“Let me say this as clearly as I possibly can, as simply and straightforward as I can: I am running … no one’s pushing me out. I’m not leaving. I’m in this race to the end, and we’re going to win”, - Biden said on a call with campaign staffers.

His remarks came after US media reports indicated that the president and his team have acknowledged that his candidacy is at risk of collapsing within days if he cannot convince the public of his fitness for office.

White House spokeswoman Karine Jean-Pierre denied those reports, insisting Biden was “clear-eyed, and he is staying in the race”.