US President Donald Trump on Monday taunted opponent Joe Biden for forgetting his name and calling him "George" just a week before the election.

Trump, 74, has often accused Biden, 77, of being senile as the two candidates battle it out ahead of the November 3 vote.

Joe Biden's habit of verbal gaffes reemerged on Sunday evening when he struggled to remember Trump's name as he addressed a virtual concert by TV link.

He twice called his opponent "George" -- perhaps a reference to one of the Bush presidents.

"Four more years of George, er, George, er, he -- we're going to find ourselves in a position where, if Trump gets elected, we're going to be in a different world," Biden said, sitting next to his wife Jill, who appeared to be prompting him.

Trump has sought to paint Biden as mentally unfit for office, and the president swiftly jumped on the moment of forgetfulness.

"Joe Biden called me George yesterday. Couldn't remember my name," Trump tweeted with glee. "The Fake News Cartel is working overtime to cover it up!"

Biden, who would be the oldest ever president if elected, has fiercely fought back at Trump's allegations over his mental health, saying they were a tactic to distract from the president's record in office.