Democrats in the US House of Representatives say they will introduce an article of impeachment against President Donald Trump on Monday, accusing him of "incitement of insurrection" for encouraging the protesters who stormed the Capitol on Wednesday as a joint session of Congress met to certify President-elect Joe Biden's victory over Trump.

House Speaker Nancy Pelosi said Sunday in a letter to colleagues that  the House of Representatives would proceed with legislation to impeach Trump. She had earlier called for his resignation or removal by Vice-President Mike Pence and the Cabinet, by means of the disability clause in the 25th Amendment to the Constitution.

"In protecting our Constituion and our Democracy, we will act with urgency, because this President represents a threat to both," the letter said.

A group of House Republicans who voted to accept Biden's Electoral College victory asked the president-elect to persuade Pelosi to back away from attempting to impeach Trump.

The lawmakers, led by Representative Ken Buck of Colorado, warned in a letter to Biden on Saturday that Trump's impeachment would inflame his supporters anew and damage Biden's efforts to unify the country.

Biden said during a news conference on Friday that it was up to Congress to decide whether to impeach Trump, but that the fastest way to remove the president would be his own inauguration on Jan 20. He said he wanted Congress to be ready the day he's sworn in to address his agenda, led by new measures to curb the pandemic and stimulate the economy.

On Friday, Pelosi said she called the chairman of the Joint Chiefs of Staff, General Mark Milley, to ask about "available precautions" to prevent Trump from initiating military action abroad or using his sole authority to launch nuclear weapons in the last days of his term. There was no comment from Milley.

Pelosi announced her call to the Democratic caucus as an effort to prevent "an unhinged president" from using the nuclear codes.

The move on Monday to attempt to impeach Trump before the end of his term on Jan 20 will come almost 11 months after he was acquitted of impeachment articles by the Republican-controlled Senate in February 2020. Only one Republican — Senator Mitt Romney of Utah — voted with Democrats in the minority to convict him.

A two-thirds majority is required for conviction in a chamber that will be evenly divided with 50 Democrats and 50 Republicans. No president has  been impeached twice.

The House could approve the impeachment article in days, perhaps with some disaffected Republicans joining the Democratic majority to send the matter to the Senate, where it was considered unlikely that 17 Republicans would go along with Democrats to reach the two-thirds necessary for conviction.

But there is the matter of time. Senator Roy Blunt of Missouri, a member of the Republican leadership, told a television station in his home state that another Trump impeachment is "not going to happen. ... There is no way we're going to impeach the president. There's not the time to do it."

Representative David Cicilline of Rhode Island drafted the new impeachment article. He was among House leaders during Trump's previous impeachment process in late 2019 on charges of abuse of power and obstruction of Congress. As of Saturday, the effort had 185 co-sponsors in a chamber where 218 votes are needed to approve an article of impeachment.

Republican Senators Pat Toomey on Sunday joined another Republican senator, Lisa Murkowski of Alaska, in calling for Trump to "resign and go away as soon as possible".

Senator Marco Rubio, a Florida Republican, said that instead of coming together, Democrats want to "talk about ridiculous things like 'Let's impeach a president' who isn't even going to be in office in about nine days."

Senate Majority Leader Mitch McConnell has circulated a letter saying that the Senate couldn't hold an impeachment trial before Trump's term ends on Jan 20 with the swearing in of Biden and Vice-President-elect Kamala Harris.

Five people died during or after protests at the Capitol, including a woman shot by police and a police officer who died Thursday after being bludgeoned.

Advocates of impeachment say they want to pursue a conviction that could prevent Trump from holding federal office again. The House article suggests that Trump's punishment warrants "removal from office, and disqualification to hold and enjoy any office of honor, trust or profit under the United States".

But the Senate would decide what punishment to hand out only after at least a two-thirds majority voted to convict Trump.

CNN, citing two sources familiar with the matter, reported on Saturday that if the Senate were to hold a trial, Trump is expected to be represented by his personal attorney, Rudy Giuliani, and is also considering hiring attorney Alan Dershowitz for his defense team.

Dershowitz, a Harvard Law School emeritus professor, said in an email on Saturday that he would defend Trump on grounds of free speech.

Agencies contributed to this story.