The US House of Representatives session was cancelled and security was high Thursday after officials warned of an attack plot by QAnon conspiracists, two months after backers of ex-president Donald Trump stormed the Capitol building.

Lawmakers and staff were advised to stay away after the FBI and Homeland Security Department warned that violent militia groups and QAnon followers had discussed attacking the legislature on or about March 4.

The FBI-Homeland Security bulletin said extremists are still motivated by unfounded Republican claims of widespread voter fraud in the November presidential election won by Democrat Joe Biden.

Also driving the possible threat is a belief by followers of the QAnon conspiracy theory that Trump could be restored to the White House on March 4, which was the date for US presidential inaugurations before 1933.

In late February some militant groups "discussed plans to take control of the US Capitol and remove Democratic lawmakers on or about March 4," according to the bulletin.

The Capitol Police echoed the bulletin with their own alert.

"We have obtained intelligence that shows a possible plot to breach the Capitol by an identified militia group on Thursday, March 4," they said Wednesday.

The Capitol was not on full lockdown, as the warnings said the threats were more "aspirational" than developed.

The Senate was to continue meeting and House Speaker Nancy Pelosi was planning a press conference in the building.

In the morning hours of Thursday there were no signs of significant protests outside the buildings of the sprawling Capitol complex.

- Extremists under investigation -

But the Senate sergeant at arms emailed senators and staff outlining enhanced security measures, including additional personnel, in place amid the fresh threat.

The new security surge came nearly two months after the January 6 breach of the Capitol in what top officials have called an unprecedented insurrection and an "attempted coup" to prevent Biden from assuming the presidency.

More than 200 have been arrested and hundreds more are under investigation, including members of extremist groups like the Proud Boys, Oath Keepers and Three Percenters who espouse ultra-nationalist, neo-nazi and white supremacist views.

A repeat of the January 6 violence, in which hundreds of Trump backers forced their way into the Capitol, shutting down the process to certify Biden as the election winner, was seen as unlikely.

Capitol Police and the National Guard were on heightened alert and the Capitol grounds are fenced off. In addition, there was no sign of the many thousands of Trump supporters who were in Washington on January 6.

But security officials warned that the most violent groups have also spoken of using explosives, with Biden's yet-to-be scheduled plan to address Congress a possible target.

Earlier this week FBI Director Christopher Wray told a Congressional hearing that his agents had 2,000 open cases dealing with domestic extremists, double the number from 2017.

While security officials have avoided tying the violence to Trump, who now lives in Florida, they say one driving force remains the unfounded claims that his 2020 election loss was due to widespread voter fraud.

Trump repeated the claim last Sunday in an address to the annual gathering of the Conservative Political Action Conference.