Employees in the US Defense Department office complex in the Arlington suburb of Washington were ordered to shelter in place for more than an hour after gunfire erupted in the station, the entrance of which is just a few dozen yards (meters) from the building's main doors.

"Multiple" people were injured, according to emergency workers, though details on the injuries were not available.

Subway services were temporarily suspended and buses headed to the station were diverted to other stops, the Washington Metropolitan Area Transit Authority.

"The Pentagon currently is on lock down due to an incident at the Pentagon Transit Center. We are asking the public to please avoid the area," the Pentagon's security force said in a tweeted statement.

Around 40 minutes later, the force said the scene of the incident was secure, without providing any details of what happened.

"It is still an active crime scene. We request that everyone stay away from the Metro rail entrance and bus platform area. Transportation at the Pentagon is diverted to Pentagon City."

Arlington County emergency services said it encountered "multiple" people needing treatment at the scene, without providing any more information.

There was a heavy presence of security and fire and rescue vehicles at the iconic five-sided building.

Pentagon spokesman John Kirby said Secretary of Defense Lloyd Austin was not in the building at the time.

Austin, according to a Pentagon source, was with President Joe Biden in a regularly scheduled meeting.