Washington DC - President Joe Bide warned that Donald Trump was "a genuine danger to American security" in his first TV interview since dropping out of the White House race.

"Mark my words, if he wins... this election, watch what happens," Biden told CBS News in a pre-taped interview broadcast on Sunday.

"He's a genuine danger to American security. Look, we're at an inflection point in world history... and democracy is the key."

The 81-year-old has kept a low profile since withdrawing after his flailing debate performance against Trump underlined fears about his age and mental abilities.

Donald Trump Trump's dictator fixation revealed in new memoir by ex-German chancellor Merkel

In the short interview recorded at the White House last week, he appeared frail but cogent, again admitting he failed in the debate but stressing that health-wise he has "no serious problem."

Explaining his exit, he said other Democratic Party politicians standing for re-election feared he would damage their chances – and added that his only priority was to stop Trump from returning to power.

"A number of my Democratic colleagues in the House and Senate thought that I was going to hurt them in the (election) races," he said. "A critical issue for me still is – not a joke – maintaining this democracy."

"I have an obligation to the country to do what is the most important thing we can do, and that is – we must, we must, we must defeat Trump."

Biden said he was proud of his record on jobs, investment, and Covid recovery, and vowed to campaign hard for Vice President Kamala Harris, who has replaced him on the ballot.

"I'm going to do whatever Kamala thinks I can do to help most," he said.

The outgoing president said he had expected to serve only one term when he won in 2020, but that he had been persuaded to stay on.

"I thought of myself as being a transition president. I can't even say how old I am. It's hard for me to get it out of my mouth, but things got moving so quickly, it didn't happen," he told CBS's Robert Costa..