Congressional Democrats were moving closer to a historic step on Friday as they weighed impeaching US President Donald Trump for a second time, two days after his false claims of election fraud helped encourage a mob that breached the US Capitol.

Top Democratic leaders, including House of Representatives Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer, called for immediate impeachment proceedings if Vice President Mike Pence and Trump's cabinet refused to take steps to remove Trump from power.

“The president's dangerous and seditious acts necessitate his immediate removal from office,” they said in a statement on Thursday evening, accusing Trump of inciting an “insurrection”.

As calls for his ouster mounted on Thursday, Trump released a video in which he denounced the violence that saw one person shot dead by police and three die from medical emergencies.

The Republican president came the closest yet to conceding his loss in the Nov 3 presidential election, promising to ensure a smooth transition to a “new administration”.

President-elect Joe Biden, a Democrat, is set to be sworn in on Jan 20.

Trump's words were in stark contrast to his speech on Wednesday, when he exhorted a crowd of thousands to descend upon the Capitol as Congress met to certify Biden's election victory.

Rioters stormed the building, overwhelming police and forcing authorities to transport lawmakers to secure locations for their own safety. A Capitol police officer died from injuries sustained in the assault, the force said late on Thursday, bringing to five the number of related deaths.

In pictures: Chaos, violence, mockery as pro-Trump mob occupies Congress

With less than two weeks left in Trump's term, it was not clear whether enough time remained to complete the impeachment process.

Pelosi has not announced a decision, though she made clear at a news conference that rank-and-file Democrats in her caucus were clamouring for action in the wake of Wednesday's siege.

If impeached in the House, Trump would theoretically face trial in the Republican-controlled Senate, which is scheduled to be in recess until Jan 19. Aides to Mitch McConnell, the majority leader, have not said what he would do if the House approves articles of impeachment.

The Democratic-led House impeached Trump in December 2019 for pressuring the president of Ukraine to investigate Biden, but the Republican-held Senate acquitted him in February 2020.

Only two other presidents in history have been impeached, and none has ever been impeached twice.

Democrats will control the Senate after sweeping two runoff elections in Georgia on Tuesday, but the new senators, Jon Ossoff and Raphael Warnock, will not be sworn in until the state certifies the results. The state deadline to do so is Jan 22, though it could come sooner.

In Thursday's video, a flat-toned Trump struck a conciliatory note seldom seen from the mercurial president, calling for “healing”.

As recently as Thursday morning, however, Trump was still claiming the election had been stolen, and he stopped short of acknowledging his loss.

Since November, Trump has baselessly railed against the election results as “rigged” due to widespread fraud.

The Trump campaign and its allies filed dozens of lawsuits challenging the vote counts but were almost universally rebuffed in state and federal courts. Election officials have said there is no evidence to back Trump's claims.

Pelosi and Schumer called on Pence and Trump's cabinet to invoke the US constitution's 25th amendment, which allows them to strip the president of his powers if he cannot discharge the duties of his office. But Pence opposes the idea, according to an adviser.

At least two Republicans, Maryland Governor Larry Hogan and US Representative Adam Kinzinger, said Trump must go. The Wall Street Journal editorial page, seen as a leading voice of the Republican establishment, on Thursday evening called on Trump to resign.

Several high-ranking Trump administration officials resigned in protest over the invasion of the Capitol, including two cabinet members: Elaine Chao, the transportation secretary and McConnell's wife, and Betsy DeVos, the education secretary.

At a news conference to introduce his pick for attorney general, Biden blamed Trump for instigating the attack but did not comment on his possible removal.

Congress certified Biden's election victory early on Thursday, after authorities cleared the Capitol. More than half of House Republicans and eight Republican senators voted to challenge some states' election results, backing Trump.

The president has isolated himself among a small circle of diehard advisers and lashed out at those he perceives as disloyal, including Pence — whom Trump wanted to try to block Congress from certifying Biden's win — according to sources.