US Capitol Police and National Guard soldiers are on high alert amid potential threats from a violent militia to storm the Congress building.

Law enforcement officials hold grave concerns about reports that 50,000 members of militia group the Three Percenters had planned to storm the Capitol on Thursday (local time), two months after the deadly riots on January 6.

It came a day after US police uncovered intelligence of the “possible plot”, which appears to be connected to a far-right conspiracy theory that former president Donald Trump will rise again to power on March 4, the original inauguration day for presidents before 1933.

Pro-Trump conspirators who believe this theory think no American president has been officially inaugurated since Ulysses S. Grant in 1869.

They claim all presidents who were elected after him are illegitimate because of a law they say had passed in 1871 that secretly turned the US into a corporation.

The intelligence had warned that the conspiracy theorists were encouraging thousands of people to flock to Washington DC on March 4 to remove Democrats from power.

The House of Representatives has cancelled its planned session on Thursday as a precaution.

There have yet to be any no signs of disturbance.

Former chairman of the House Homeland Security Committee, Michael McCaul, was among those briefed about the new threat.

“I think we’ll see some violence here,” he said.

About 5200 National Guard troops have joined Capitol Police officers inside a massive black fence that surrounds the Capitol grounds and several neighbouring buildings.

Capitol Police are responding to what the force called “a possible plot to breach the Capitol”.

On one of the warmest days in weeks, the National Mall was almost totally deserted on Thursday except for joggers, journalists and a handful of tourists trying to take photos of the Capitol dome through the fence.

House Speaker Nancy Pelosi said the Guard troops protecting the Capitol should stay as long as they are needed amid the new threat of another mob attack.

-with AAP