We must end this uncivil war, Biden says

Joe Biden has been sworn in as president of the United States, offering a message of unity and restoration to a deeply divided country reeling from a battered economy and a raging coronavirus pandemic that has killed more than 400,000 Americans.

Standing on the steps of the US Capitol two weeks after a mob of then-president Donald Trump's supporters stormed the building, Biden called for a return to civic decency in an inaugural address marking the end of Trump's tempestuous four-year term.

"To overcome these challenges, to restore the soul and secure the future of America, requires so much more than words. It requires the most elusive of all things in a democracy: unity," Biden, a Democrat, said on Wednesday after taking the oath of office.

"We must end this uncivil war that pits red against blue, rural versus urban, conservative versus liberal. We can do this - if we open our souls instead of hardening our hearts."

Saying there was "no time to waste", Biden signed 15 executive actions shortly after entering the White House to set a new course and overturn some of the most controversial policies of Trump, a Republican.

The orders included mandating masks on federal property, halting the withdrawal from the World Health Organisation, rejoining the Paris climate accord and ending a travel ban on some Muslim-majority countries.

Biden told reporters in the Oval Office that Trump had left him "a very generous letter", but he would not disclose its contents.

The inauguration served as a stark reminder of the tumult that defined the Trump era as well as the pandemic that still threatens the country.

Amid warnings of possible renewed violence, thousands of armed National Guard troops circled the Capitol. The National Mall, typically packed with supporters, instead was filled with nearly 200,000 US flags. Dignitaries - including former US presidents Barack Obama, George W Bush and Bill Clinton - wore masks and sat metres apart.

Biden's running mate, Kamala Harris, the daughter of immigrants from Jamaica and India, became the first black person, first woman and first Asian American to serve as vice-president.

The president spoke forcefully about the January 6 Capitol siege when Trump backers breached the building, sending lawmakers fleeing for safety and leaving five dead, including a police officer. But Biden never mentioned his predecessor by name.

The violence prompted the Democratic-controlled US House of Representatives to impeach Trump for an unprecedented second time, accusing him of incitement after he exhorted his backers to march on the building to press false claims of election fraud.

"Here we stand, just days after a riotous mob thought they could use violence to silence the will of the people, to stop the work on our democracy, to drive us from this sacred ground," Biden said. "It did not happen; it will never happen. Not today, not tomorrow, not ever."

The impeachment trial is yet to be held in the Senate.

The norm-defying Trump, who never conceded the November 3 election, flouted one last convention when he refused to meet Biden or attend the inauguration.

Trump did not mention Biden by name in his final remarks as president on Wednesday morning, when he touted his administration's record before flying to his Mar-a-Lago retreat in Florida.

Top Republicans, including vice-president Mike Pence and the party's congressional leaders, skipped Trump's send-off and attended Biden's inauguration instead.

Biden's speech struck a conciliatory tone rarely heard from Trump. He asked Americans who did not vote for him to give him a chance.

He also promised to repair foreign alliances frayed by Trump's unpredictable tenure and act as a strong partner for peace, progress and security.

Biden later attended a wreath-laying ceremony at the Tomb of the Unknown Soldier in Arlington National Cemetery, accompanied by Obama, Bush and Clinton.

Afterward, his motorcade joined a parade on his way to the White House. Biden and his family left their limousines to walk the final few hundred metres to their new home.

In her first news briefing, White House spokeswoman Jen Psaki said Biden's first call with a foreign leader would be with Canadian Prime Minister Justin Trudeau on Friday. They would discuss Biden's decision to revoke the permit needed to build the Keystone XL oil pipeline and other issues.

Biden's top priority is a $US1.9 trillion ($A2.5 trillion) stimulus plan to alleviate the financial pain from coronavirus. But it will require approval from a divided Congress, where Democrats hold slim advantages in the House and Senate.

At 78, Biden is the oldest US president in history. The 46th president's career includes more than three decades in the US Senate and two terms as vice-president under Obama.