WASHINGTON: US President Joe Biden defended both his "mental acuity" and his rhetoric about Donald Trump on Monday, in a second TV interview aimed at ending calls for him to quit his reelection bid following a disastrous debate.

The 81-year-old delivered an often combative defence of his mental and physical fitness for office during the one-on-one with broadcaster NBC, which nevertheless featured some of the word salads that have worried Democrats.

"I'm old," Biden told host Lester Holt in the interview at the White House. "But I'm only three years older than Trump, number one. And number two, my mental acuity has been pretty damn good."

He added: "I understand why people say, 'God, he's 81 years old. Whoa. What's he gonna be when he's 83 years old, 84 years?' It's a legitimate question to ask."

Biden's interview was the latest attempt by the White House to assuage growing fears over the Democrat's age and mental state following the dismal debate performance on June 27.

The sit-down was announced last week, before the assassination attempt on Republican candidate Donald Trump on Saturday - which inevitably became a key subject of the interview.

Biden told Holt he was wrong when he recently told donors it was "time to put Trump in the bullseye" of his election campaign.

"It was a mistake to use the word," Biden said when asked if he had gone too far with his rhetoric as a deeply polarised nation reels from the shooting, in which Trump's ear was injured.

"I meant focus on him, focus on what he's doing," Biden said.

But Biden doubled down on what he said was the need to "talk about the threat to democracy" posed by former president Trump.

"Look, I'm not the guy that said 'I want to be a dictator on Day One'," he said, referring to remarks by Trump that alarmed many people.

Several prominent Republicans, including Trump's newly announced running mate J.D. Vance, have since accused Biden of having responsibility for the assassination attempt because of his language against Trump.

The allegations - themselves often incendiary - come as authorities have said they've yet to identify the shooter's ideology, and even as Republicans themselves often use guns in their political imagery.

The interview screened just an hour before Trump took to the stage with a bandage on his ear at the Republican National Convention in Milwaukee, his first public appearance since the shooting.

While the attempt to kill Trump has taken some of the heat off Biden over his debate performance, the Democrat's every move remains closely scrutinised amid calls for him to step aside.

The primetime interview with the major NBC network follows a previous sit-down with ABC anchor George Stephnopoulos and a "big boy" press conference at a Nato summit last week.

But Biden, who added to his long history of gaffes by mixing up the names of the Ukrainian and Russian leaders at the summit, also struggled for words on a number of occasions on Monday and let several sentences trail off.

While trying to portray a statesmanlike image, Biden often showed flashes of irritation.

Asked if the Trump shooting had changed the trajectory of the election, Biden replied: "I don't know, and you don't know either."

Biden sparred with interviewer Holt, asking "why don't you guys ever talk about the 18-20 lies he (Trump) told" during the debate.

He also reacted sharply when asked if he would "get back on the horse" and add a third debate, replying: "I'm on the horse, where have you been?"

Biden confirmed that he would still take part in the second debate in September.

 - AFP