The Pentagon's top generals condemned the January 6 attack on the Capitol by Trump supporters on Tuesday, telling US troops that it was an illegal assault on the constitutional process.

"The violent riot... was as direct assault on the US Congress, the Capitol building, and our Constitutional process," said a memorandum signed by all eight members of the Joint Chiefs of Staff, led by the chairman, General Mark Milley.

"The rights of freedom of speech and assembly do not give anyone the right to resort to violence, sedition and insurrection," they said.

The letter said members of the armed services are bound to defend the constitution.

"Any act to disrupt the constitutional process is not only against our traditions, values and oath; it is against the law."

The message came amid concerns that the Trump-supporting extremists who raided the Congress to halt the certification of Joe Biden as the next president have supporters in the armed forces and law enforcement.

Trump and his supporters have refused to accept that Biden fairly and soundly won the November 3 presidential election. 

The Pentagon is deploying as many as 15,000 National Guard troops to protect Biden's inauguration on January 20, amid fears of new violence.

Pentagon officials were asked Monday about the possibility of pro-Trump activists in the Guard and among regular troops.

"We don't tolerate extremists in our ranks," said spokesman Jonathan Hoffman.

Underscoring the point, the Joint Chiefs said that, "in accordance with the Constitution," Biden would be inaugurated on January 20th "and will become our 46th commander in chief."

In South Korea, the commander of US Forces Korea, General Robert Abrams, retweeted the JCS statement saying there was "no ambiguity" the events at the Capitol were an "attempted insurrection". 

"If you are serving in uniform and think it was something else, I would encourage (you) to sit down and read the constitution that you swore an oath to support and defend," wrote Abrams, who uses the handle @DogFaceSoldier. 

"No room on our team if you are not willing to defend the constitution against all enemies, foreign AND domestic."

Troops carry arms on Washington streets

National Guard troops deployed on the streets of Washington began carry weapons late Tuesday in a major change of posture ahead of Joe Biden's inauguration as US president, according to an AFP photographer.

The Guard soldiers were originally mobilized to provide mostly logistical support to Washington police, and on Monday General Daniel Hokanson, Pentagon National Guard Bureau chief, said they had not been authorized yet to carry weapons.

Authorizing Guard members to deploy in a law enforcement role, armed and empowered to make arrests, would be a "last resort" if the security situation got out of hand, Hokanson said.

It was not clear what changed late Tuesday, and the city's National Guard had no comment.

Security experts have said chatter among extremists and supporters of President Donald Trump on social media about holding armed marches and threatening violence in the US capital and other cities had surged in recent days.

The Pentagon is mobilizing up to 15,000 National Guard for the January 20 inauguration, as Trump and supporters still refuse to accept Biden's election victory two months ago.

Five people died on January 6 when Trump supporters attacked and shut down the US Congress in an attempt to halt the certification of Biden's election win.

At the time Guard troops were nearby, but were unarmed and called in too late to back up the Capitol Police to protect the legislature.

The New York Times reported that weapons had been authorized Tuesday for the troops guarding the Capitol complex.