Trump, who has not appeared in public since a violent mob of his supporters besieged the Capitol on Wednesday, will be the first incumbent president since Andrew Johnson not to attend his successor's inauguration.

Biden said he was just fine with that, calling it "one of the few things we have ever agreed on".

"It's a good thing him not showing up," Biden said, calling the president an "embarrassment" to the nation and unworthy of the office.

Traditionally, the incoming and outgoing presidents ride to the US Capitol together for the ceremony, a visible manifestation of the smooth change of leadership.

Biden will become president at noon on January 20 regardless of Trump's plans.

But Trump's absence represents one final act of defiance of the norms and traditions of Washington that he has flouted for four years.

Historian Douglas Brinkley said that while attending the inauguration "would be a wonderful olive branch to the country if Trump would do it, and it might be his chance at one magnanimous act on his way out of office, he is unable to do that".

While Trump stays away, former presidents Barack Obama, George W Bush and Bill Clinton and their wives will be there.

The only other living president, 96-year-old Jimmy Carter, who has spent the pandemic largely at home in Georgia, will not attend but has extended "best wishes" to Biden.

Trump's tweet on Friday that he would boycott the inauguration came as he holed up in the White House with a dwindling coterie of aides and as momentum grew on Capitol Hill to subject him to impeachment for a second time.

"To all of those who have asked, I will not be going to the Inauguration on January 20th," Trump tweeted.

It was not a surprise: Trump for more than two months has falsely claimed he won re-election and advanced baseless claims of widespread voter fraud, even though his own administration has said the election was fairly run.

Senator Rick Scott, a Florida Republican, urged Trump to reconsider.

"He is, of course, not constitutionally required to attend and I can imagine losing an election is very hard, but I believe he should attend," Scott said in a statement.

The senator said he planned to attend and called the rite "an important tradition that demonstrates the peaceful transfer of power to our people and to the world".

Vice President Mike Pence, who defied Trump on Wednesday when he refused to intervene in the congressional process to certify Biden's win, was expected to attend the inauguration, according to one person close to him and one familiar with inauguration planning.

But Pence spokesman Devin O'Malley said in a statement Friday that the vice president and the second lady "have yet to make a decision regarding their attendance".

Biden said Pence was "welcome to come" and he'd be honoured to have him there.

Australian Associated Press