Washington neared a wartime footing Thursday as it heightened security for President-elect Joe Biden's inauguration Jan 20, two weeks after the storming of the Capitol by supporters of President Donald Trump.

Hotels, airlines and other businesses ramped up security as authorities planned to deploy at least 26,000 National Guard troops to protect the US capital in the days leading to the Jan 20 inauguration. 

FBI Director Christopher 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".

The perimeter of a high fence surrounding the Capitol has been extended to encompass the US Supreme Court and the Library of Congress.

Most of the District of Columbia's downtown was off limits to traffic, with one journalist comparing it to Baghdad's high security "Green Zone".

The city announced that 13 Metro stations inside the security perimeter will be closed for several days.

The National Park Service said a decision had yet to be made on whether to close the National Mall running from the Capitol to the Lincoln Memorial. The Park Service closed the Washington Monument to tours, and Washington Mayor Muriel Bowser asked visitors to stay away from the city.

On Wednesday, the US House of Representatives voted to impeach Trump on charges of incitement a week after his supporters rampaged through the Capitol, resulting in five deaths. The incursion disrupted the vote of a joint session of Congress to certify Biden's Electoral College victory.

In his first public appearance since the Jan 6 attack, Vice-President Mike Pence, who The New York Times reported met with Wray on Thursday, 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."

Delta, Alaska, American, and United Airlines announced Thursday they will not allow travelers flying to Washington-area airports to check firearms before the inauguration.

On Wednesday, Airbnb announced it was canceling its 5,500 reservations in the Washington metro area. 

"We are continuing our work to ensure hate group members are not part of the Airbnb community," the company said in a statement.

Pentagon officials approved requests to have some National Guard troops carry either long guns or handguns, particularly those assigned near the Capitol building. 

"In deciding to arm the National Guard troops assigned to the Capitol complex for President-elect Joseph R. Biden Jr.'s inauguration, the Defense Department crossed a Rubicon that for the last six months Pentagon officials have tried to avoid: potentially pitting armed military forces against American citizens in the streets," The New York Times reported.

"And it is some of the very same people — Democrats — who have in the past warned against a muscular response to past protests, now pushing for an armed military." 

Guard members will carry M9 sidearms and some will have automatic rifles and shotguns. 

The Times, citing people familiar with conversations among Biden transition officials, said the team is "deeply anxious about what would happen if a shooting breaks out on Inauguration Day or the days leading up to it".

The decision Tuesday night by the Army secretary to arm some of the National Guard members was reached after a meeting with House Speaker Nancy Pelosi, Democrat of California.

Pelosi, according to congressional staff members, demanded that the Pentagon "take a more muscular and proactive posture to the inauguration than it did last week", the Times reported.

Sixteen groups, some saying they will be armed and most of them staunch Trump supporters, have registered to hold protests in Washington, officials said.

On Wednesday, Biden received a briefing from FBI officials, the Secret Service and his national security team.

"In the week since the attack on Congress by a mob that included domestic terrorists and violent extremists, the nation has continued to learn more about the threat to our democracy and about the potential for additional violence in the coming days, both in the National Capital Region and in cities across the country," said a statement from the Biden transition team. 

Biden, 78, will no longer be taking an Amtrak train, his long-favored means of transportation, into Washington for his inauguration because of security concerns, a person briefed on the decision told The Associated Press on Wednesday. 

Federal prosecutors have charged more than 70 people in the Capitol attack, including some who appeared in viral photos and videos. Officials expect to eventually charge hundreds of others.

A prosecutor says a retired Air Force officer who participated in the siege carried plastic zip-tie handcuffs because he meant "to take hostages".

Retired lieutenant colonel Larry Rendall Brock Jr, 53, appeared at a detention hearing in Fort Worth, Texas, on Thursday. He is charged with knowingly entering or remaining in a restricted building or grounds without lawful authority, and violent entry and disorderly conduct on Capitol grounds. A judge said he would release Brock to home confinement.

Brock's attorney says there is no evidence that he did anything violent inside the Capitol.

More than 30 House Democrats, including lawmaker Mikie Sherrill, have asked the acting House and Senate sergeants-at-arms and the acting head of the Capitol Police for visitor logs, videos and other information about who was in the building that day.

Sherrill said she saw other House members leading groups on "reconnaissance" tours of the Capitol the day before the attack.

Peter Meijer, one of 10 House Republicans who voted with Democrats on Wednesday to impeach Trump, said he and other lawmakers were taking precautions like wearing body armor.

"I have colleagues who are now traveling with armed escorts out of the fear for their safety," he told MSNBC. "Our expectation is that someone may try to kill us."

Two members of Congress called Thursday for the Army to send cots and other equipment for National Guard troops who have been sleeping on floors in the halls of the Capitol, politico.com reported.

Representatives Rosa DeLauro of Connecticut and Betty McCollum of Minnesota, in a letter to Army Secretary Ryan McCarthy, said they were "disappointed" by images that went viral of soldiers resting in the Capitol Rotunda.

"With the uncertainty for needed rest and recoup time in flux, and to ensure that the Guard members are fully able to execute their protection mission, we urge you to make available cots or other equipment to more easily facilitate their ability to rest while they are on Capitol grounds," wrote DeLauro, chair of the House Appropriations Committee, along with McCollum, the ranking Democrat on the panel that funds the military.

Agencies contributed to this story.