US President Joe Biden has blamed his poor performance at last week's debate with Donald Trump on exhaustion from international travel.

The 81-year-old president faced calls to step aside after he delivered rambling answers in a raspy voice and appeared to lose his train of thought at times.

"The fact is that you know, I wasn't very smart. I decided to travel around the world a couple times, going through around 100 time zones ... before ... the debate. Didn't listen to my staff and came back and nearly fell asleep on stage," Biden said at a fundraiser on Tuesday.

"That's no excuse but it is an explanation."

Biden's recent travels include separate trips to D-Day commemorations in France and the G7 meeting in Italy, as well as time in California, Washington D.C. and Camp David.

Harris 'proud' to be Biden's running mate

Biden's comments come after Vice President Kamala Harris said she was proud to be on the ticket with him.

"I am proud to be Joe Biden's running mate," Harris said in an interview with CBS News.

"Joe Biden is our nominee. We beat Trump once and we're going to beat him again," Harris added.

White House Press Secretary Karine Jean-Pierre also said that Biden had no intention of stepping aside after what she described as his "bad night" at the debate.

"We really want to turn the page on this," she said.

zc/jsi (AP, AFP, Reuters)