"He knew it - he knew it and did nothing," Biden told a CNN town hall in Pennsylvania on Thursday.

"It's close to criminal."

The broadcast was the first town hall-style event that Biden has done since accepting the Democratic nomination for president last month, giving viewers a rare chance to see him answer questions from people whose votes he hopes to win in November.

The cable network described the event, in Biden's birthplace of Scranton, as a "drive-in town hall": participants remained parked in their cars outside a minor league baseball stadium to ensure they stayed safely distanced from one another.

For much of the summer, Biden held mostly virtual events from his home in Delaware, drawing criticism from Trump that he was "hiding".

But Biden maintained an advantage over Trump in national opinion polls throughout, as the pandemic and subsequent economic downturn battered Trump's standing among voters.

In recent weeks, Biden has begun to travel again to swing states for campaign appearances, but they have been largely devoid of attendees, aside from reporters and invited guests in a nod to the ongoing pandemic.

Trump, who has returned to holding large rallies both indoors and outside, participated in a town hall on the US's ABC TV network with undecided Pennsylvania voters earlier this week, where he defended his administration's response to the outbreak despite acknowledging to the journalist Bob Woodward in March that he had deliberately downplayed the virus's deadliness.

COVID-19, the disease caused by the virus, has killed more than 195,000 and infected more than 6.3 million people in the United States, the most of any country.

Biden and Trump will both travel to Minnesota on Friday, the first day of early voting there.

Australian Associated Press