Joe Biden has said he "nearly fell asleep" during last week's first presidential debate and put his poor performance down to travel exhaustion after going "around the world a couple of times" shortly before it.

The US president, 81, said he "wasn't very smart" for having travelled extensively in the weeks leading up to the event in Atlanta, where he performed so poorly that friends and foes alike have told him to quit the race for the White House.

Mr Biden told supporters at a campaign event in Virginia on Tuesday: "I decided to travel around the world a couple of times, going through around 100 time zones."

He added he "didn't listen" to his staff, "came back and nearly fell asleep on stage".

"That's no excuse but it is an explanation," he added while admitting: "I didn't have my best night."

The Biden campaign doesn't allow media outlets to film fundraisers, so it wasn't immediately clear whether he was joking, NBC, Sky News' US partner said.

The president visited France and Italy during two separate trips in the space of two weeks last month, flying overnight from the G7 summit in Bari to appear at a fundraiser with former president Barack Obama in Los Angeles on 15 June before returning to Washington the following day.

However, he then spent six days at Camp David preparing for Thursday's contest against presumptive Republican nominee, Donald Trump.

But there was little sign of preparation as the Democrat candidate paused and stumbled his way through a gruelling TV debate in a performance dismissed as an "unmitigated disaster" by some in his own party.

White House officials have blamed his halting showing on a cold, but Mr Biden did not mention illness.

In contrast, the 78-year-old Trump, who has long said Mr Biden is too old for another term, appeared vigorous and lucid, albeit while repeating what critics have said are oft-repeated false claims.

Meanwhile, calls are growing from prominent Democrats for Mr Biden to withdraw from the 2024 presidential race.

Lloyd Doggett, a Democrat member of Congress representing Texas, said in a statement: "Our overriding consideration must be who has the best hope of saving our democracy from an authoritarian takeover by a criminal and his gang. Too much is at stake to risk a Trump victory."

Former House Speaker Nancy Pelosi also told MSNBC that she believes "it is a legitimate question" whether Mr Biden's performance was just "an episode or is this a condition".

She said: "When people ask that question, it's legitimate - of both candidates."

However, a spokesman later said Mrs Pelosi had full confidence in Mr Biden and "looks forward to attending his inauguration on 20 January 2025".

It comes as a poll published on Tuesday showed that one in three Democrats think Mr Biden should end his re-election bid following the debate.

But the research by Reuters/Ipsos also suggested the Democrats would be unlikely to do much better if they changed their candidate and Mr Biden has said he will not quit.

The two-day poll found that both he and Trump have the backing of around 40% of registered voters, meaning the president may not have lost ground since the debate, despite the negative reaction.