[Ammi]

A gunman has been "pinned down" after he shot more than ten people at the US Naval Sea Systems Command headquarters in Washington.

The gunman, said to have been carrying an AR-15 rifle, launched his attack on one of the biggest naval command buildings in the US in the early busy morning rush as people arrived at the naval base for work.

US defence officials have said that several people have been killed and as many as 10 wounded.

Some US media were reporting that there were three gunmen involved in the attack. However, this has not been confirmed by the authorities.

Law enforcement officials suggested at one gunman was "contained".

Police and federal agents from multiple law enforcement agencies responded as the base, where 3,000 people work, was put on lockdown while officers searched for the shooter inside the complex.

Ambulances were parked outside, streets in the area were closed and flights at Reagan National Airport were temporarily halted.

The President is receiving frequent briefings on the grave and unfolding situation.

Police have confirmed that at least 10 people have been shot. Details of their condition have not been disclosed.

Sky News sources say that two were law enforcement officers

Sky News US Correspondent Greg Milam said: "Varying reports about how many people may have been caught up in this… possibly as many as 10.

"One report it was a black male wearing black clothing and carrying an assault rifle-type weapon.

"They are bringing in more and more police resources."

A Navy statement said: "An active shooter was reported inside the ... building on the Washington Navy Yard at 8:20 am Eastern Time.

"Emergency personnel remain on scene and a 'shelter in place' order has been issued for Navy Yard personnel.

"This release will be updated as new information is confirmed."

Police from multiple local and federal agencies along with SWAT crews are attending the emergency

Fox News reporter Peter Doocy said: "It is frantic right now … people are running full steam … all over the place.

"I am looking at what appears to be an all-black armoured vehicle.

"(There are) sirens, ambulances, police cars. Men in full fatigues, with long guns, running down the street.

"Everybody you can think of is here."

A photograph said to have been taken in the yard shows one man lying on the ground and being treated by members of the public.

Naval Sea Systems Command is the largest of the Navy's five system commands and accounts for a quarter of the Navy's entire budget.

It builds, buys and maintains the Navy's ships and submarines and their combat systems