Joe Biden blames foreign travel for debate disaster, says he almost fell asleep on stage

Beleaguered US President Joe Biden has apologised for his lacklustre performance at last week's debate and said his recent overseas trips were to blame.

The 81-year-old's faltering display against Republican challenger Donald Trump did nothing to quell concerns he has neither the mental or physical capacities for another four years in the White House.

Biden acknowledged on Tuesday (Wednesday AEDT) that he "wasn't very smart" for having travelled extensively in the weeks ahead of the debate in Atlanta, reports Reuters.

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"I decided to travel around the world a couple of times … shortly before the debate," Biden said at a campaign fundraiser in McLean, Virginia.

"I didn't listen to my staff … and then I almost fell asleep onstage," he quipped.

"It's not an excuse but an explanation," he added.

Advisers to Biden previously said he had a cold during the debate

It was unclear whether he was referring to falling asleep at Democrat fundraisers he attended after the debate.

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Cameras are not permitted at his fundraisers, and the media relies on written pool reports.

On multiple past occasions, Biden has made candid or surprise comments at fundraisers.

The US President last month visited France to commemorate the 80th anniversary of D-Day. He then returned to the US before he left for the Group of Seven Summit in Italy.

Biden then visited Los Angeles for a fundraiser and later spent several days at Camp David in Maryland preparing for the debate.