Employees in the US Defense Department office complex in Arlington, a suburb of Washington, were ordered to shelter in place for more than an hour after gunfire erupted in the bus and subway station just yards (meters) from the Pentagon entrance.

Officials said the site was secure 90 minutes after the incident but declined to explain what happened.

"The incident is is over, the scene is secure, and most importantly there's no continuing threat to our community," said Woodrow Kusse, chief of the Pentagon Force Protection Agency, which partols the iconic five-sided building.

Kusse said there were "several injuries" but would not confirm reports that one of his officers was stabbed and died from their injuries, and that the attacker was shot dead.

Other officials offered condolences over the reported death.

"I'm incredibly saddened to learn about the passing of a Pentagon police officer, who was killed earlier this morning in a senseless act of violence outside the Pentagon," said Senator Mark Warner, who chairs the Senate Intelligence Committee.

Scores of police gathered outside the George Washington University Hospital in Washington where the officer was taken.

Subway services were temporarily suspended and buses headed to the station were diverted to other stops.

"The Pentagon currently is on lockdown due to an incident at the Pentagon Transit Center. We are asking the public to please avoid the area," the Pentagon's security force tweeted at 10:37 am (1437 GMT).

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

Pentagon spokesman John Kirby said Secretary of Defense Lloyd Austin was out of the building at the time, in a meeting with President Joe Biden.