Joe Biden is offering an explanation for his lacklustre performance while debating Donald Trump.

The US President says his travel schedule had him running on fumes, and feeling sleepy, so he almost fell asleep on the debate stage.

Biden made the excuse Tuesday, July 2, at a campaign fundraiser in McLean, Virginia.

He blamed his lack of energy on being exhausted from long trips in the weeks before Thursday's first presidential debate, including a trip to France in early June for the 80th anniversary of D-Day.

Biden told the room, “I decided to travel around the world a couple of times ... shortly before the debate ... I didn't listen to my staff ... and then I almost fell asleep on stage.”

Biden went to France the first week of June, flew to California for a big fundraiser in Los Angeles on June 15 and then spent a week preparing for the June 27 debate at Camp David.

Biden told supporters he was not providing an excuse but an explanation. He went on to apologize for his debate performance while stressing the importance of beating Trump at the ballot box.