US President Joe Biden says he dropped out of his re-election bid because he feared that the intraparty battle over his candidacy would be a "real distraction" for Democrats and that his highest priority was to defeat Donald Trump in November.

Axar.az reports that in his first interview since quitting the race, Mr Biden, 81, said he had “no serious problem” with his health. He blamed his poor debate performance on being sick at the time and brushed off concerns about his age and mental acuity.

The US president pledged to campaign for Kamala Harris saying he was going to do whatever his vice-president "thinks I can do to help most".

“We must, we must, we must defeat Trump,” he told US broadcaster CBS News.

Mr. Biden said if he had continued his campaign, the presidential contest would have gone “down to the wire”.

"A number of my Democratic colleagues in the House and Senate thought that I was going to hurt them in the races," he said.

"And I was concerned if I stayed in the race, that would be the topic. You’d be interviewing me about, Why did Nancy Pelosi say, why did so — and I thought it’d be a real distraction.”