The rivals spoke in simultaneous town halls broadcast on separate television networks after a debate originally scheduled for Thursday was called off following Trump's COVID-19 diagnosis.

Biden, speaking to voters in Philadelphia on ABC, blamed the Republican president for concealing the deadliness of the virus that has killed more than 217,000 people in the United States.

"He said he didn't tell anybody because he was afraid Americans would panic," Biden said on Thursday night. "Americans don't panic. He panicked."

Trump defended both his response to the pandemic as well as his own personal conduct, including staging a Rose Garden event at the White House where few wore masks or practised social distancing, which resulted in numerous attendees contracting the disease.

"Hey, I'm president - I have to see people, I can't be in a basement," Trump said on NBC in front of an outdoor audience in Miami, implicitly criticising Biden for spending months off the campaign trail as the pandemic raged.

He did not answer questions about the last time he tested negative before getting the virus, saying he did not recall precisely.

Trump, who aggressively interrupted Biden during a chaotic debate two weeks ago, showed little interest in altering his belligerent tone, sparring frequently with moderator Samantha Guthrie.

He said he "heard different stories" about the efficacy of masks, even though his own administration's public health experts have said wearing them is key to stopping the spread of the virus.

The president declined to denounce QAnon, the false conspiracy theory that Democrats are part of a global pedophile ring, first praising its adherents for opposing pedophilia before saying he knew nothing about the movement.

Trump also dodged questions about a New York Times investigation of two decades of his tax returns, which he has refused to release publicly.

The second presidential debate had originally been scheduled for Thursday but Trump pulled out after organisers decided to make it virtual following his diagnosis two weeks ago. A final debate is still scheduled for October 22 in Nashville, Tennessee.

Trump, who spent three days in a military hospital but has since returned to the campaign trail, is trying to alter the dynamics of the race. Reuters/Ipsos polls show Biden has a significant national lead, although his advantage in battleground states is less pronounced.

About 18.3 million Americans have voted either in person or by mail so far, representing 12.9 per cent of the total votes counted in the 2016 election, according to the US Elections Project at the University of Florida.

Voters are seeking to avoid queues on election day to stay safe as coronavirus infections continue to rise but also to make sure their ballots will count. Many are concerned Trump will challenge widely used mail-in ballots after his repeated claims without evidence that they were fraudulent.

The Senate confirmation hearings for Amy Coney Barrett, Trump's nominee for a vacant Supreme Court seat, prompted careful answers from both candidates. Republicans have pushed to seat Barrett before the election after refusing to do so for former Democratic president Barack Obama's nominee in 2016, saying it was inappropriate in an election year.

Biden said he was not a "fan" of court-packing, the idea promoted by some Democrats of adding justices to countermand what they view as a stolen seat. But he declined to rule it out after moderator George Stephanopoulos pressed him.

Trump would not say whether he would like to see abortion rights invalidated, an outcome that many legal scholars believe is more likely with the conservative Barrett on the court.

Both candidates have visited battleground states this week, with Trump holding rallies in Florida, Pennsylvania and Iowa and Biden travelling to Ohio and Florida.

Australian Associated Press