Gunshots have been fired near the entrance to the Pentagon, resulting in multiple injuries.
The facility, the headquarters of the US military, was temporarily placed on lockdown.
Arlington County Fire Department reported "multiple patients" but it was not clear if they had been shot and the extent of their injuries were unknown.
A police officer was among those injured, according to two law enforcement officials.
The incident occurred on a Metro bus platform that is part of the Pentagon Transit Centre, the Pentagon Force Protection Agency tweeted.
The facility is just steps from the Pentagon building, which is in Arlington County, Virginia, just across the Potomac river from Washington DC.
An Associated Press reporter near the building heard multiple gunshots, then a pause, then at least one additional shot.
Another AP journalist heard police yelling “shooter”.
A Pentagon announcement said the facility was on lockdown due to “police activity”.
The agency responsible for security at the building, the Pentagon Force Protection Agency, tweeted shortly before noon (5pm Irish time) that the scene of the incident was secure.
The lockdown was lifted except for the area around the crime scene.
Defence secretary Lloyd Austin and Gen Mark Milley, chairman of the Joint Chiefs of Staff, were at the White House meeting US president Joe Biden at the time of the shooting.
In 2010, two officers with the Pentagon Force Protection Agency were wounded when a gunman approached them at a security screening area.
The officers, who survived, returned fire, fatally wounding the gunman, identified as John Patrick Bedell. – AP