WASHINGTON (REUTERS) - Overwhelmed during the deadly attack on Congress by President Donald Trump's supporters last week, US security forces are mounting a national operation to thwart any violence before President-elect Joe Biden takes office.

Federal and state officials are evaluating online threats and menacing messages to members of Congress and making sure the security operation has the force to repel an attack.

The incoming Biden administration has significantly ramped up security around his team ahead of the Jan 20 inauguration.

The riot at the Capitol in Washington on Jan 6 forced lawmakers to flee the inner chambers of the building, fearing for their lives.

One police officer and four protesters died in the siege, which began after Mr Trump called on thousands of supporters to march on Congress in a bid to stop the final certification of Mr Biden's election victory.

Mr Trump, who falsely claims he lost the election because of fraud, was impeached in the House of Representatives on Wednesday (Jan 13) for encouraging his supporters to march on Congress.

He has also faced growing calls to resign.

The Federal Bureau of Investigation has warned that armed protests in support of Mr Trump were planned for Washington and all 50 US state capitals this weekend or around Mr Biden's inauguration.

Local officials have been ramping up security, especially in the battleground states where Mr Biden narrowly beat Mr Trump.

"It is clear that more must be done to preempt, penetrate, and prevent deadly and seditious assaults by domestic violent extremists in the days ahead," two senior House Democrats Jerrold Nadler and Adam Schiff said in a statement on Tuesday.

The biggest single security operation is in Washington, where Mr Biden will be sworn in outside the Capitol next Wednesday.

Some 20,000 National Guard troops will be available for Mr Biden's inauguration, and half of those will be in the city by Saturday in case protests explode this weekend.

National Guard troops were assigned to provide 24-hour security inside the halls of Congress and deputised as Capitol Police officers, enabling them to make arrests.

The National Park Service has closed the Washington Monument through Jan 24, and home-sharing company Airbnb said it was blocking and cancelling all reservations in the Washington area to discourage protesters from descending on the capital.

In Wisconsin, Governor Tony Evers authorised National Guard troops to support security efforts in the state capital of Madison. Georgia's top court cancelled all in-person proceedings on Jan 19 and Jan 20 as a precaution.

"While individual state capitals have been subject to threats and actual violence across the years, the current environment is unique at least since the weeks following Sept 11, 2001," said Mr James Nash, spokesman for the bipartisan National Governors Association.

Conflicting messages have surfaced in far-right chat rooms and forums about possible protests around Mr Biden's inauguration.

The website of Patriot Action for America, which was recently taken down, called for supporters to encircle the White House, Congress and Supreme Court days before "to, at all costs, prevent Joseph Biden, or any other democrat from being inaugurated".

An image posted to the pro-Trump website The Donald called for protesters to "stand up for liberty" with armed marches on Jan 17 at the US Capitol and in state capitals, according to SITE Intelligence Group, a Maryland-based organisation that tracks extremists.

But users on some other far-right forums cautioned people not to attend the events, which they warned might be part of a plot to ensnare them.

Mr Trump has defended his Jan 6 speech to supporters before they marched to Congress as "totally appropriate", but he had a different message on Wednesday as public criticism from his fellow Republicans grew.

"In the light of reports of more demonstrations, I urge that there must be NO violence, NO lawbreaking and NO vandalism of any kind," Mr Trump said in a statement.

Acting US Homeland Security Deputy Secretary Ken Cuccinelli on Tuesday appeared to downplay the threat at state capitols.

"There are a lot of statements out there. And while we need to take them seriously, at the same time it would be an incredible unlikelihood to see what is described in those threats actually take place," Mr Cuccinelli said in an interview with Fox News Channel.

Still, a congressional source familiar with intelligence reporting and analysis said the FBI is "taking pretty seriously"the warnings of Inauguration Day protests.

Multiple investigations are under way into how Mr Trump's supporters were able to storm the Capitol building, and whether some public officials participated.

At least two Capitol Police officers have been suspended and more than a dozen others are under investigation for alleged dereliction of duty, aiding or abetting rioters. Off-duty police alleged to have participated in the riot have also been suspended by departments outside Washington.

Dozens of cases are being investigated for potential charges ranging from sedition to felony murder related to the riot, in what acting (Washington) DC Attorney-General Sherwin called a significantcounter-terrorism probe.