US presidential candidate Joe Biden says Donald Trump might try to “steal” the November election and refuse to leave the White House if he loses.

Appearing on The Daily Show, Mr Biden speculated his rival might cheat to stay in power. He said he had considered what would happen if Mr Trump refused to vacate the presidency.

The Democrat presidential nominee suggested the military could step in to ensure a peaceful transition of power.

“I am absolutely convinced they will escort him from the White House with great dispatch,” the former vice-president said on Friday (Australian time).

But White House press secretary Kayleigh McEnany said that was “a ridiculous proposition” and accused Mr Biden of trafficking in “conspiracy theories”.

“This president’s looking forward to November,” Ms McEnany told Fox News Channel’s America’s Newsroom.

“This president’s hard at work for the American people. And leave it to Democrats to go out there and grandstand and level these conspiracy theories.”

As Mr Biden’s comments were aired, fears of a second coronavirus wave caused the US stock market to plummet to levels not seen since the height of the pandemic.

It came less than a day after the US recorded more than 2 million cases of the virus, making investors concerned about the economic impact of a potential second wave.

On Friday morning (Australian time), the Dow Jones dropped 1862 points. All three major indexes posted their biggest single-day declines since March 16.

“We are still in a very deep recession. We still have a tremendous amount of uncertainty about the virus but also about stimulus and politics,” David Kelly, chief global market strategist at JPMorgan Funds, told CNN.

Regret over walk with Trump

Top US military officer Mark Milley has admitted it was a “mistake” for him to accompany Mr Trump on his controversial walk to a photo opportunity at a historic church during the George Floyd protests.

“I should not have been there,” General Milley said on Friday morning (Australian time), almost two weeks after he was photographed in his combat uniform with the presidential entourage.

Mr Trump’s entourage included, from left, Attorney-General William Barr, Secretary of Defence Mark Esper and General Mark Milley. Photo: AAP

Riot police used tear gas to clear peaceful protesters moments before the President’s now-infamous walk through Lafayette Park near the White House to pose outside St John’s Church with a Bible in his hands.

General Milley, who chairs the Joint Chiefs of Staff, said he regretted joining Mr Trump at the height of the protests about Mr Floyd’s death – and minutes after Mr Trump said he would deploy the military if states and cities could not quell the demonstrations.

In remarks to a National Defence University commencement ceremony, General Milley said his presence and the photographs ultimately compromised his commitment to a military divorced from politics.

“My presence in that moment and in that environment created a perception of the military involved in domestic politics,” he said.

“As a commissioned uniformed officer, it was a mistake that I have learned from, and I sincerely hope we all can learn from it.

“We must hold dear the principle of an apolitical military that is so deeply rooted in the very essence of our republic.”

-with AAP