Delta, Alaska, American, and United Airlines announced on Thursday they would not allow travellers flying to Washington area airports to check firearms on its flights before the inauguration.

Delta chief executive Ed Bastian told CNBC his airline had placed passengers on a no-fly list for their involvement in disruptive incidents that, for example, targeted Republican US Senator Mitt Romney.

National railroad service Amtrak also said it would implement heightened security measures, including the deployment of additional police officers on trains.

The actions follow the storming of the US Capitol last week by supporters of President Donald Trump and disturbances on flights and at airports. Officials have warned of plans for armed protests in Washington and all 50 states.

In his first public appearance since the January 6 attack, Vice-President Mike Pence said: "We're going to ensure that we have a safe inauguration and that President-elect Joe Biden, Vice-President-elect Kamala Harris are sworn in as the new president and vice-president."

FBI director Christopher Wray said the bureau was seeing an "extensive amount of concerning online chatter" including calls for armed protests in the coming week.

Online comments were being analysed and investigated for their actual threat potential as law enforcement tried to distinguish between aspirational and intentional calls.

Wray said he was "concerned about the potential for violence at multiple protests and rallies planned here in DC and at state capitol buildings around the country".

In Washington, the perimeter of a high fence surrounding the Capitol was pushed out to encompass the US Supreme Court and the Library of Congress.

Nearby roads were closed, some businesses said they would shut down next week, and the city's public transportation agency said it was closing certain metro stations and rerouting bus lines from Friday through January 21.

The National Park Service has closed the Washington Monument to tours and is deciding whether to close the National Mall running from the Capitol to the Lincoln Memorial. Mayor Muriel Bowser has asked visitors to stay away from the city.

Law enforcement officials have warned of potential violence beyond the US capital. In Ohio, Governor Mike DeWine said the statehouse and state office buildings in downtown Columbus would be closed from Sunday through Wednesday.

Mary McCord, head of the Justice Department's national security division under former president Barack Obama, warned the threat could remain well beyond the inauguration.

The US House of Representatives on Wednesday voted to impeach Trump on charges of incitement after his supporters rampaged through the Capitol, leaving five dead, following a speech by the Republican president reiterating his false claim that Biden, a Democrat, beat him because of widespread fraud.

Federal authorities have arrested dozens of people as part of their investigation into the Capitol assault.

Australian Associated Press