US President Joe Biden has admitted that he "almost fell asleep on stage" and "was not very smart" enough to handle his schedule before the first presidential debate with Donald Trump, blaming his extensive foreign travels for his disastrous performance.

Biden, 81, seeking a second term in the White House, stumbled in his first televised presidential debate with his predecessor Trump, 78, on Thursday night last week, setting off alarm bells among top Democrats about whether the incumbent president can stay atop in the gruelling months ahead of the elections.

During their roughly 90-minute debate defined by personal attacks, Trump, the presumptive Republican Party candidate in the November 5 presidential election, clashed right from the start with Biden, arguing pointedly about the American economy, foreign relations and migration.

“I wasn't very smart. I decided to travel around the world a couple of times...shortly before the debate,” Biden said while talking to donors at a fundraiser in a Virginia suburb of Washington DC on Tuesday.

“I didn't listen to my staff...and then I almost fell asleep on stage,” he said, amidst pressure on Biden to quit the race for the White House due to his advancing age.

Biden admitted that he did not have a good debate, and apologised by saying that he was sorry for the performance.

"It’s not an excuse but an explanation," he said.

The president’s remarks at the fundraiser lasted just six minutes, which is far shorter than what he speaks on such occasions.

Meanwhile, the White House also acknowledged on Tuesday that President Biden did not have a “great (debate) night”, but asserted that he knew how to get things done and is ready to run the country for the next four years.

“Honestly, this is something that the president has addressed himself multiple times since this past Thursday. First of all, I want to say, we understand the concerns. We get it. The president did not have a great night. As you all know and many of you reached out during the debate, the president had a cold. He had a hoarse voice,” White House Press Secretary Karine Jean-Pierre told reporters at her daily news conference.

“You all heard it, that's why you reached out. But I will say this, and the president said this over the past couple of days, certainly right after the debate. He knows how to do the job and he knows how to do the job, not because he says it, because his record proves it. Because for three and a half years, almost four years, the president's record has been unprecedented, delivering for the American people,” she said.

Jean-Pierre was flooded with questions about Biden’s health, his status of mental health, and his ability to run the country for the next four years.

“Another thing that he said that I would add is he knows right from wrong. He knows how to tell the truth. And again, he knows how to deliver for the American people. Joe Biden is a person, take away his title, he is someone who has dealt with tragedy. He is someone who has confronted that head-on. He is someone who knows how to get back up once you've been knocked down,” she said.

“That is something that he understands very, very well and I think, and we believe that's something that many Americans across the country understand as well. He knows how to come back. He knows how to come back. So, the president is going to continue to focus on what he's been doing the past three and a half years,” Jean-Pierre said.

Biden, she said, is going to continue to focus on the American people. She quoted the remarks of former House Speaker Nancy Pelosi that it is not about performance in terms of a debate, it's about performance in a presidency.

"This is a president, you've heard me say this, we've talked about this. He's been able to give us an economic recovery, the strongest economic recovery in modern history. Let's not forget, during the midterms of 2022, many people, many of you all, some of you in this room said that it was going to be a red wave and that didn't happen. He defeated Big Pharma,” Jean-Pierre said.

"We have seen a historic low in crime in more than down to 50 year low and then you had Representative Clyburn who said the president has done a great job leading for the last three and a half years.

"The best predictor of future behaviour is past performance. This is from Clyburn. And when you look at the record of President Biden versus former President Trump, you see President Biden who has delivered the strongest recovery in modern history versus the previous administration whose plans hurt the middle class," she said.

Jean-Pierre said it is not uncommon for incumbents to have a bad night on their first debate. "I think the president's record certainly speaks for itself," she said.

Biden will travel to Wisconsin on Friday where he will have a sit-down interview with George Stephanopoulos from ABC News, while on the campaign trail.

He will travel to Philadelphia on Sunday. And next week, he will host a NATO press conference, Jean-Pierre said.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.