Employees in the US Defense Department headquarters in the Arlington suburb of Washington were ordered to shelter in place amid reports of several gunshots and possible injuries in the station, the entrance of which is just a few dozen yards (meters) from the building's main doors.

"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.

The local news station WUSA showed a picture of heavy security and fire and rescue vehicles at the iconic five-sided building.

There was no immediate report of injuries.