U.S. President Joe Biden today blamed jet lag from his recent travels overseas for his disastrous debate performance against Republican rival Donald Trump.

"It's not an excuse, but an explanation," he said while addressing a fundraiser.

Biden said he "wasn't very smart" for "traveling around the world a couple times... shortly before the debate."

"I didn't listen to my staff," he said. "And then I almost fell asleep on stage."

Biden had flown to France, back to the United States, to Italy, among other travels, over a 14-day period before taking just a few days to rest at his vacation home in Rehoboth Beach, Delaware. He was dragging, according to several people who observed him during this period.

Joe Biden, who has always been gaffe-prone, appeared to struggle while responding to former US President Donald Trump's charges during the debate.

A bombastic Trump lashed out at his successor, calling him a failure on the economy and the world stage. Biden looked to hit back, but his delivery was hesitant as he spoke rapidly in a soft, trailing-off voice and stumbled on his words several times.

It was the first debate ever between a president and former president -- and each accused each other of being history's worst.

The 81-year-old's fumbling, halting performance has sparked calls from Democrats for him to end his quest for a second term and for "soul-searching".

Biden's remarks could invite even more scrutiny over whether the 81-year-old is fit for rigorous traveling, packed schedule.

White House press secretary Karine Jean-Pierre today admitted the debate was "a bad night" but added that Biden "knows how to come back" from adversity.

Responding to the criticism, US Secretary of State Antony Blinken said people around the world were looking at what Biden has done since coming into office, not just one night, and that they appreciate his policies.