American law enforcement officials are hunting two more gunmen after a deadly rampage at a key naval installation in Washington DC.

DC police chief Cathy Lanier confirmed that one shooter had died after he was "engaged" by security forces at the Naval Sea Systems Command headquarters, where 3,000 are employed.

Lanier said that security believed that another two gunmen were involved part in the shootings which had led to "multiple" deaths.

Officials later confirmed that at least seven people had been killed and 12 wounded. One of the deceased was a police officer.

"We potentially have two other shooters we have not located," Lanier said.

Her reports seemed to contradict earlier claims by the authorities that a second gunman was "down".

She said that both suspects were wearing military-style uniforms. It is not known if they were past or serving military personnel.

Both suspects, one black, in his 50s, and one white, are male. The white suspect was seen holding a handgun while the second had a long gun, Lanier added.

Washington mayor Vincent Gray said that four wounded victims were taken to hospital.

Witnesses reported that a gunman had been standing at the window on the fourth floor of a building and firing at people in the cafeteria three storeys below.

President Barack Obama and defence secretary Chuck Hagel were briefed on the situation.

"We are confronting yet another mass shooting," said Obama, pledging to hold accountable those responsible for the "cowardly act".

He praised staff at the Navy yard.

"Today, they faced the unimaginable violence they wouldn't have expected at home," he said.