Biden gave a presidential-style speech on Wednesday acknowledging people's fatigue with restrictions amid the pandemic but urging them to exercise caution as caseloads surge.

"I know the country has grown weary of the fight," Biden said as he urged Americans to forgo the type of big family gatherings normally associated with Thursday's holiday, wear protective masks and socially distance.

"We need to remember - we're at war with the virus, not with one another."

The Democratic former vice president again said he would take immediate steps to address the coronavirus pandemic on taking office on January 20.

Biden accused Trump during the campaign of panicking and surrendering in the face of a public health crisis.

Shortly after Biden spoke, Trump pardoned his former national security adviser Michael Flynn, who twice pleaded guilty to lying to the FBI about his contacts with the former Russian ambassador in Washington.

It marked the latest instance in which Trump used his power of executive clemency to benefit a friend or associate.

More than 261,000 Americans have died from COVID-19, with the daily toll on Tuesday surpassing 2000 for the first time since May, as infections and hospitalisations surge nationally.

The United States leads the world in COVID-19 cases and deaths.

Biden said the United States faced "a long hard winter" but that it was during the most difficult circumstances that "the soul of our nation has been forged".

He said he hoped the recent positive news on vaccine development - the first shots potentially could be made available to some Americans within weeks - would serve as incentive for people to take simple steps to get the virus under control.

Since winning the November 3 election, Biden has offered a message of national healing and reconciliation after Trump's tumultuous term, while the Republican president still refuses to concede and falsely claimed again on Wednesday the election was stolen.

Without mentioning Trump, Biden addressed the messy election aftermath.

"Our democracy was tested this year," Biden said.

"In America, we have full and fair and free elections and then we honour the results. The people of this nation and the laws of the land won't stand for anything else."

Biden plans next week to name his choices for some key positions in his administration, including his economic team, communications director Kate Bedingfield said.

They are expected to include former Federal Reserve Chair Janet Yellen as Treasury secretary.

Trump's administration gave the green light on Monday to formal transition efforts even as he continues to challenge the election results.

Biden was expected to receive his first presidential daily intelligence briefing on Monday, Bedingfield said.

Bedingfield added Biden's team had been encouraged by the "professional and welcoming response" of civil servants.

Trump has waged a failed legal battle to overturn the election results.

The outgoing president on Wednesday cancelled a trip with personal attorney Rudy Giuliani to a meeting of Republican legislators in Pennsylvania, where Giuliani repeated his unsubstantiated allegations of voting fraud.

But Trump spoke to the participants by speaker phone, repeating his debunked claims and drawing cheers from the partisan crowd.

Australian Associated Press