As the US House of Representatives opened its impeachment hearing on Wednesday, the District of Columbia National Guard said it was authorised to arm troops assigned to security duty on US Capitol grounds.
The Guard said the authorisation was requested by federal authorities and approved by Army Secretary Ryan McCarthy about 6pm on Tuesday.
Up to 15,000 Guard members are expected to be on duty in the coming days to support law enforcement in connection with the inauguration of president-elect Joe Biden on January 20.
Authorities are concerned about threats of violence, after the insurrection at the Capitol last week.
The Pentagon's top generals condemned the January 6 attack by supporters of President Donald Trump on Tuesday, telling US troops that it was an illegal assault on the constitutional process.
"The violent riot was a 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, Gen Mark Milley.
"The rights of freedom of speech and assembly do not give anyone the right to resort to violence, sedition and insurrection."
The letter said members of the armed services were 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," it said.