US President Joe Biden has vowed to stay in the presidential race despite growing calls for him to step aside.

Worries have grown following Biden's shaky performance in a televised debate against his Republican rival, former President Donald Trump.

Biden gave an interview with ABC News on Friday, his first TV interview after last month's closely watched debate. His performance there sparked calls even from within his Democratic Party for him to withdraw from the November election.

In the interview, Biden said he had a bad cold the day of the debate. He also said that when he was answering a question, Trump was still shouting, which Biden said distracted him.

Pressed by the interviewer about whether he has the mental and physical capacity to serve effectively for another four years, Biden said, "I wouldn't be running if I didn't think I did."

Biden boasted about his achievements during his three-and-a-half years in office, citing the strong US economy in particular.

He said, "I don't think anybody's more qualified to be president or win this race than me."

US media outlets quoted some Democrats as saying Biden gave a more forceful performance than during the debate. But they also reported that the interview fell short of allaying concerns about his age.

Attention remains focused on whether Biden will be able to quell calls for withdrawal from within his own party.