US President Joe Biden has suggested that he could defeat Donald Trump again in a 2024 rematch, and warned that the former Republican president was a threat to the country.

One day after launching his re-election bid, President Biden marked the occasion on Wednesday not on the campaign trail but in a Rose Garden news conference following a meeting with South Korean President Yoon Suk Yeol.

A reporter pressed the Democratic president on whether he was running for re-election because he believed he was the only one who could beat Mr Trump.

“I may not be the only one, but I know him well and I know the danger he presents to our democracy,” President Biden said.

“We’ve been down this road before.”

President Biden’s Tuesday launch video included imagery from the January 6, 2021, attack on the US Capitol by Mr Trump’s supporters and included pledges to protect American liberties from “extremists” linked to the former president.

President Biden said he ran in 2020 because of what he described as a unique threat posed by Mr Trump. Now, he says his decision to run again isn’t connected to Mr Trump.

“I think I still would be running if he wasn’t,” President Biden said of Mr Trump, who has already announced his candidacy, along with several other Republican hopefuls.

President Biden added that he took a “hard look” at whether his age, 80, should factor into whether he decided to run and that he expected voters to look fairly at the same issue.

“I took a hard look before I decided to run and I feel good, I feel excited about the prospects and I think we’re on the verge of really turning the corner in a way we haven’t in a long time.”

The president said there is more he needs to do to position the United States so it is “economically and politically secure for a long time.”

– AAP