The US House of Representatives will proceed with legislation to impeach Donald Trump, speaker Nancy Pelosi has said.

"In protecting our constitution and our democracy, we will act with urgency, because this president represents an imminent threat to both," the House speaker wrote to colleagues in a letter on Sunday.

"The horror of the ongoing assault on our democracy perpetrated by this president is intensified and so is the immediate need for action."

Mrs Pelosi said the House will first try to get Mr Trump's Vice President Mike Pence and his cabinet to oust him using the 25th amendment with legislation on Monday.

But as this will almost certainly be blocked by Republicans, a full House vote will be convened on Tuesday to pave way for the impeachment process.

If passed by the House, the impeachment articles would be taken to the Senate for a trial, with senators acting as jurors who would ultimately vote on whether to acquit or convict Mr Trump.

A number of Republicans would have to join the Democrats in voting to impeach him, which requires a two-thirds majority.

But Democrat momentum to impeach the president for a second time has already gained senior Republican support.

Pennsylvania senator Pat Toomey said he believes the president committed "impeachable offences" by inciting the violence that led to the deadly Capitol riots that left five people dead.

He joined his fellow senator Lisa Murkowski of Alaska in demanding the president's resignation.

There are now 200 co-sponsors for the impeachment legislation that Democratic representative Ted Lieu plans to introduce on Monday.

But although the Democrats want to be quick to publicly condemn Mr Trump's actions, they are thought to be delaying any impeachment trial for at least 100 days.

This would allow president-elect Joe Biden to establish himself and set out his priorities following his inauguration on 20 January.

It would also stop Mr Trump from ever running for the presidency again.

Senate majority leader Mitch McConnell has said there is not enough time for an impeachment trial ahead of Mr Biden's inauguration.

Meanwhile Mr Trump has become increasingly isolated, holed up in the White House, and abandoned by aides in the aftermath of last week's violence.

A string of resignations followed the scenes in Washington, including two cabinet members both of whom are women.

His remaining aides hope a trip to Alamo, Texas, on Tuesday will allow the president to highlight the policy accomplishments of his administration.

He is expected to talk about his efforts to curb illegal immigration and build a border wall during the visit.

The countdown of Mr Trump's final days in office come as authorities attempt to identify all those who stormed the Capitol on Wednesday, with some off-duty police officers and firefighters thought to be among them.

Police departments in Virginia and Washington state have placed officers on leave, while they examine whether they took part in unlawful acts while away from work.

And fire departments in Florida and New York City have reported to federal authorities allegations that some of their members may also have been present.

Five people, including a Capitol police officer, died when the protesters broke into the building as Congress met to certify the results of the presidential election.

The crowd surged to the domed symbol of American democracy following a rally near the White House, where the outgoing president repeated his false and unproven claims that the election was stolen from him - and urged his supporters to march in force toward the Capitol.

The president has few fellow Republicans speaking out in his defence in wake of the chaos, with the former governor of California and Hollywood legend Arnold Schwarzenegger among those to have condemned him.