US president Donald Trump has refused to publicly commit to accepting the results of the upcoming White House election, recalling a similar threat he made weeks before the 2016 vote, as he scoffed at polls showing him lagging behind his Democratic rival Joe Biden.

Trump said it is too early to make such an iron-clad guarantee. "Look ... I have to see," he told moderator Chris Wallace during an extraordinary interview on Fox News.

The president added: "No, I'm not going to just say 'yes'. I'm not going to say 'no', and I didn't last time either."

The Biden campaign responded: "The American people will decide this election. And the United States government is perfectly capable of escorting trespassers out of the White House."

Trump also hammered the Pentagon for favouring the renaming of bases that honour Confederate military leaders – a drive for change spurred by the national debate about race after George Floyd's death.

"I don't care what the military says," the American commander in chief said.

The president described America's top infectious diseases expert, Dr Anthony Fauci, as a "a little bit of an alarmist" about the coronavirus pandemic, and stuck to what he had said back in February - that the virus is "going to disappear".

On Fox, Trump said: "I'll be right eventually." The United States tops the global death toll list with more than 140,000 and confirmed infections, with 3.7 million.

It is remarkable that a sitting president would express less than complete confidence in the American democracy's electoral process.

But for Trump, it stems from his tactics of four years ago, when in the closing stages of his race against Hillary Clinton, he said he would not commit to honouring the election results if the Democrat won.

Pressed during an October 2016 debate about whether he would abide by the voters' will, Trump responded that he would "keep you in suspense".

The president's remarks to Fox are certain to fuel conversation on Capitol Hill, where members of congress had already been airing concerns in private about a scenario in which Trump disputes the election results.

The president has seen his presidential popularity erode over his handling of the coronavirus pandemic and in the aftermath of nationwide protests centred on racial injustice that erupted after Floyd's death in Minneapolis nearly two months ago.

The president contends that a series of polls that show his popularity eroding and Biden holding an advantage are faulty. He believes Republican voters are under-represented in such surveys.

"First of all, I'm not losing, because those are fake polls," Trump said in the recorded interview, which aired on Sunday.

"They were fake in 2016 and now they're even more fake. The polls were much worse in 2016."

Trump was frequently combative with Wallace in defending his administration's response to the pandemic, weighing in on the Black Lives Matter movement and trying to portray Biden, the presumptive Democratic nominee, as lacking the mental prowess to serve as president.

Among the issues discussed was the push for wholesale changes in policing that has swept across the nation. Trump said he could understand why Black Americans are upset about how police use force disproportionately against them.

"Of course I do. Of course I do," the president said, adding his usual refrain that "whites are also killed, too".

He said he was "not offended either by Black Lives Matter," but at the same time defended the Confederate flag, a symbol of the racism of the past, and said those who "proudly have their Confederate flags, they're not talking about racism".

"They love their flag, it represents the South, they like the South. That's freedom of speech. And you know, the whole thing with 'cancel culture,' we can't cancel our whole history. We can't forget that the North and the South fought. We have to remember that, otherwise we'll end up fighting again. You can't just cancel all," Trump said.

Wallace challenged Trump on some of his claims and called out the president at times, such as when Trump falsely asserted that "Biden wants to defund the police".

The former vice president has not joined with activists rallying behind that banner. He has proposed more money for police, conditioned to improvements in their practices.

Trump continues to insist that Biden "signed a charter" with one of his primary rivals on the left, senator Bernie Sanders of Vermont. At one point in the interview, Trump called on aides to bring him documentation to support his assertion.

Trump, however, was unable to point to language from a Biden-Sanders task force policy document released this month by the Biden campaign.

The 74-year-old president also stuck to a campaign charge that Biden, 77, is unable to handle the rigours of the White House because of his age.

As for polls showing the incumbent is trailing, Trump noted he was thought to be behind for much of the 2016 contest. "I won't lose," he predicted.