I'll not run if I could not do the job - Biden on calls to step aside, vows to fight on

US President Joe Biden has hit back at criticism over his age, telling supporters in a fiery speech that he will win re-election in November after a poor debate performance fueled concern about his candidacy.

"I know I'm not a young man, to state the obvious," he told a rally in the battleground state of North Carolina on Friday, one day after he struggled in the televised showdown with his Republican rival Donald Trump.

"I don’t walk as easy as I used to... I don’t debate as well as I used to," he acknowledged. "But I know what I do know, I know how to tell the truth [and] I know how to do this job."

Mr Biden, 81, said he believed with his "heart and soul" that he could serve another term, as the cheering crowd in Raleigh chanted “four more years”.

While questions over Mr Biden's age are not new, his shaky performance on the debate stage - which was marked by verbal blanks, a hoarse voice and some difficult-to-follow answers - triggered panic among some Democrats who raised fresh questions about his candidacy.

Mr Biden's campaign has remained emphatic that the country's oldest presidential candidate is still fit to serve another term. Campaign officials said he would not step aside for another nominee, despite the poor performance.

"Absolutely not," Mia Ehrenberg, a Biden campaign spokesperson, said in response to questions about whether Mr Biden would vacate his candidacy.

And while several campaign surrogates - including Vice President Kamala Harris - admitted in interviews that the president stumbled during the debate, they reiterated their commitment to the elder statesman and stressed that his answers were more substantive than Mr Trump's.