A 34-year-old gunman has opened fire at a US Navy Yard in Washington in a shooting that left 13 people dead.

The suspect, identified by the FBI as Aaron Alexis of Fort Worth, Texas, was among the dead.

Authorities said they were searching for another possible gunman wearing military-style clothing.

Earlier, officials said they were looking for two men, but then confirmed police had established one of them was not a suspect in the shooting.

The Washington DC mayor and the police chief reported 13 fatalities and an unknown number of wounded at the Naval Sea Systems Command.

"We have no indication of motive at this time," said Washington DC police chief Cathy Lanier.

A Navy official told Reuters the suspected shooter was discharged from Navy reserves in 2011 after series of “misconduct issues.”

While US Navy Secretary, Ray Mabus said Aaron Alexis was at some point a civilian contractor in the area of information technology.

"At one point, he was a Navy contractor. Whether he was today, I don't know," Mr Mabus told CNN.

Reached by telephone, Mr Alexis' father, Algernon Alexis, seemed stunned by the news his son may have been involved.

He said his son was in the military and now studying while working in a computer-related job for a private company.

The Navy complex is one of five such commands where civilians, military personnel and contractors build, buy and maintain Navy ships and submarines.

About 3,000 people work there.

Patricia Ward, who works at the complex said she heard three shots "pow, pow, pow" and then four more shots after a pause.

"Everybody was panicking and trying to decide which way to get out," Ms Ward told reporters.

US reacts to another mass shooting

President Barack Obama said "We are confronting yet another mass shooting, and today it happened at another military installation, in our nation's capital."

"They know the dangers of serving abroad, but today they face the unimaginable violence that they wouldn't have expected here at home," Mr Obama said.

The Washington shooting happened less than three weeks after US Army psychiatrist Major Nidal Hasan was sentenced to death for murdering 13 people in 2009 at Fort Hood, Texas.

He later called the shooting of soldiers retaliation for US wars in Muslim countries.

Among the injured in the Washington shooting was a District of Columbia police officer, who was in stable condition in surgery, and one other law enforcement officer, officials said.

Washington Hospital Center said it was treating three gunshot victims, who were in critical condition, and was expecting more to arrive.

At George Washington University, a man in his 60s died of a single gunshot wound to his head, said Babak Sarani, head of trauma surgery.

The man was shot in his left temple and the injury "was not survivable by any stretch," Mr Sarani told reporters, adding he was unsure what type of weapon was used.

Security at naval yard breached

The command where the shooting took place requires two separate identification badges, one to get on the base and another to access the building, according to a source who works at the Navy Yard.

Military personnel are generally banned from carrying weapons on military installations but most people with proper credentials are not routinely checked for firearms.

"It will be interesting to see as this develops who the shooter is, how he got in," said Navy Commander Tim Jirus, who was in charge of evacuating the Sea Command building.

"Right now a lot of people are wondering just how safe the building is or just how safe the office environment is."

The Federal Aviation Administration said it briefly suspended departures at Reagan National Airport. The District of Columbia Public Schools put six schools and an administration building on lockdown as a precaution.

The Washington Nationals baseball team postponed their game against the Atlanta Braves scheduled for Monday night at nearby Nationals Park.