US Democrats plan to introduce an article of impeachment against President Donald Trump for his role in Wednesday's invasion of the US Capitol.

House Speaker Nancy Pelosi said she would move forward with impeachment if Mr Trump did not resign immediately.

The charge of "incitement of insurrection" is set to be introduced by House Democrats on Monday.

They accuse Mr Trump of encouraging a riot in Congress in which five people died.

President-elect Joe Biden said impeachment was for Congress to decide, but said he had thought "for a long time President Trump was not fit to hold the job".

The White House dismissed the impeachment as a "politically motivated" move that would "only serve to further divide our great country".

Nearly 160 House of Representatives Democrats have signed on to the bill, which congressmen Ted Lieu of California and David Cicilline of Rhode Island began drafting while they were sheltering in place during Wednesday's chaos at the Capitol.

If the process does go ahead, it would be the second time the House has pursued impeachment against President Trump.

In December 2019, the lower chamber impeached Mr Trump on charges of abuse of power and obstruction of Congress. But the Senate acquitted him on both charges in February 2020.

No US president has ever been impeached twice. However, the prospect of an impeachment conviction seems remote because of Mr Trump's Republican broad support in the Senate.

One moderate Republican senator, Lisa Murkowski of Alaska, told the Anchorage Daily News on Friday that Trump simply "needs to get out". And Republican Senator Ben Sasse of Nebraska, a regular critic of Trump, said he would "definitely consider" impeachment.

But there is so far no indication that enough members of the president's party would agree to convict him.

That means an impeachment in the House might only be a symbolic action to hold Mr Trump accountable for the invasion of Congress.

An internal Senate memo says the soonest it could take up any articles of impeachment from the House would be on 19 January, the day before Mr Trump's term expires, and a trial could only begin after he had left office.

Constitutional experts are split on whether impeachment can still proceed to a Senate trial in this event.

If convicted, Mr Trump would lose benefits granted to former presidents, and senators could vote to bar him permanently from public office.

The unprecedented move comes as an isolated Mr Trump holed up at the White House on Friday and his main means of communication with the outside world, Twitter, banned him from its platform.

The siege of the Capitol has put senior politicians on edge, prompting the Democratic House Speaker Pelosi to talk to the nation's top military officer about ways to prevent Mr Trump from accessing nuclear codes.

In a dramatic day in Washington DC on Friday, other developments included:

• Mr Trump has said he would not attend the inauguration of his successor, Mr Biden, on 20 January
• The FBI joined an investigation into the death of police officer Brian Sicknick from injuries sustained during the siege of the Capitol
• Dozens of people were arrested and several charged in relation to the assault on the Capitol building
• One of Mrs Pelosi's staff members revealed that a laptop was stolen from her office during the mob invasion
• Democrats and several prominent Republicans reiterated their calls for Mr Trump's cabinet to invoke constitutional powers and remove him from office.

What does the draft impeachment resolution say?

The draft resolution, which has been shared with BBC partner CBS News, consists of one article: "incitement of insurrection".

"Donald John Trump engaged in high Crimes and Misdemeanours by willfully inciting violence against the Government of the United States," the draft reads.

In the resolution, the lawmakers accuse the president of making statements that encouraged and resulted in "imminent lawless action at the Capitol".

The draft also says this was "consistent with his prior efforts to subvert and obstruct" the certification of President-elect Biden's win in November's presidential election.

"He thereby betrayed his trust as President, to the manifest injury of the people of the United States," the draft says.

The president urged his supporters to march on the Capitol on the day Mr Biden's electoral college victory was to be confirmed by Congress.

He held a rally at which he repeatedly cast doubt on the legitimacy of the presidential election, alleging, without evidence, that it was "stolen" from him.

"We're going to walk down to the Capitol and we're going to cheer on our brave senators and Congressmen and women," Mr Trump told the crowd, exhorting his supporters "to fight".

Democrats and some Republicans have argued that these words amounted to incitement.

What is impeachment?

Impeachment allows Congress - the part of the US government that writes and brings in laws - to put presidents on trial.

Articles of impeachment are charges brought against a president by the House of Representatives. If the House votes to pass them, proceedings move to the Senate, which decides whether or not to convict.

It is a rare event and a political process, rather than a criminal one.

Mr Trump was the third US president to have been impeached.

The two others, Bill Clinton in 1999 and Andrew Johnson in 1868, were left in office by the Senate.

President Richard Nixon resigned before he could be impeached.

How did we get here?

On Wednesday, with Washington still reeling from the Capitol riot, Democrats held a conference call in which they talked for three hours about the prospect of removing the president from office.

CBS News reports that Democrats were overwhelmingly supportive of the effort, with just one, Oregon Representative Kurt Schrader, arguing that such a move would be too divisive for the country.

Mrs Pelosi told her members that her preference would be to see Mr Trump removed by the 25th Amendment, which allows the vice-president to step up if the president is unable to perform his duties owing to a mental or physical illness.

If that did not happen, impeachment would be on the table, she said.

US media reports say Treasury Secretary Steven Mnuchin has been personally involved in discussions about invoking the 25th Amendment.

However, such a course of action is said to be highly unlikely, and there is no sign that Vice-President Pence is prepared to use the amendment.

On Friday, Mrs Pelosi called Mr Trump "unhinged" and said Congress must do everything possible to protect Americans while he is in office.

In a later statement, Ms Pelosi said the House Democratic Caucus had a "sad, moving and patriotic" discussion about the possibility of impeachment.