Four people have been killed and at least eight others injured in a shooting at the US Navy Yard in Washington, a Navy official told reporters.

Police were searching the Navy complex after the shooting on Monday morning to track down one or several gunmen involved in the attack.

Washington Post reported that there were three shooters, including one in military fatigues.

A Navy official said one shooter had been "contained".

Al Jazeera's Kimberly Halkett, reporting from the scene, said there was an "enormous police presence" in the vicinity of the Navy Yard. Two helicopters were flying over the complex and roads were blocked.

The Navy did not officially confirm the number of casualties, but said "several" people had suffered injuries. It said people inside the building had been directed to stay in place.

The Navy said at least three shots were fired at 8:20 am (13:20 GMT) in the headquarters building of the Naval Sea Systems Command.

About 3,000 people work at the headquarters.

'Fortress'

Witnesses saw police with rifles running towards the complex as police believed the attacker was holed up inside.

Al Jazeera's Rosiland Jordan described the attack as "shocking", since the Navy complex is a high-security facility.

"It's a fortress," she said, adding that staff need to present ID cards to get in and that cars are checked to get access to the complex grounds.

Flights at Reagan National Airport were temporarily halted to enable helicopters to fly injured to the hospital.

The White House said President Barack Obama had been briefed about the unfolding situation by senior aides.

Naval Sea Systems Command builds, buys and maintains the Navy's ships and submarines and their combat systems.

It is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.

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