At least 13 people including a gunman have been killed and a police officer wounded in a mass shooting at the US Navy Yard in Washington, DC, just three kilometres from the Capitol building.

Police are hunting for one other possible suspect, although there is still confusion about reports of multiple shooters at the scene.

Officials said at least three people were wounded in the rampage, including the officer, but all three were expected to recover.

US President Barack Obama said he was mourning "yet another mass shooting" and called it a "cowardly act".

Authorities have not said if they believe the attack was an act of terrorism, but a massive police operation is underway that includes “active shooter units” of the DC Metropolitan Police and the Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as officers from the FBI, the US Park Police, the NCIS and Homeland Security.

The dead gunman has been identified as 34-year-old civilian contractor Aaron Alexis, originally from Fort Worth, Texas. His identity was confirmed by photographic ID found on his body.

He is believed to have a criminal record and hold a concealed carry weapon permit.

Alexis is believed to have accessed the Navy Yard by using someone else’s identification card. It is not yet clear if that individual was an accomplice or if that person’s ID card was stolen.

According to Navy officials, Alexis enlisted in the Navy in 2007 and left the service in 2011. He served in the Fleet Logistics Support Squadron (46) at the Naval Air Station in Fort Worth.

"He didn't seem aggressive to me"

A man who identified himself as Alexis' ''best friend'' said he was shocked to learn that Alexis was the suspect.

"He lived with me three years," Nutpisit Suthamtewakul told the Star-Telegram in Forth Worth. ''I don't think he'd do this. He has a gun, but I don't think he's that stupid. He didn't seem aggressive to me."

His former landlord in Fort Worth, Somsak Srisan, said Alexis was always well-mannered.

"Oh boy, I can't believe this," Mr Srisan told the newspaper. "He was always very polite to me. When he lived at my house, I never see him get angry about anything. My feeling is if he was angry about anything, he didn't show that to me."

Mr Srisan said he had seen Alexis meditate and he had always remained calm around him.

"I can't believe he would do anything like this," Mr Srisan said.

Staff told to take shelter

The first reports of an incident came through just after 8.15am (10.15pm AEST), when the DC Metropolitan Police received calls saying there was a gunman shooting at the Washington Navy Yard. The complex is home to both the Chief of Naval Operations and the Navy Systems Command, the administrative group in charge of designing and building US warships and their naval weapons systems.

As fire alarms sounded, the 3000 civilian and military employees were ordered to “shelter in place”. Metropolitan Police Chief Cathy Lanier said the department’s first active shooter response team was on site within seven minutes.

One witness reported hearing three shots fired rapidly followed by another four shots, leading to speculation that a semi-automatic firearm was in use.

Commander Tim Jirus said he had been standing talking with a man when the man was shot in the head and fell to the ground, apparently dead. Commander Jirus had just helped evacuate staff from his building and had been waiting at a mustering point when the man had come out of a nearby maintenance building to ask him what was going on.

“He just came up and was like, ‘Hey I understand you have a shooter in your building’,” Commander Jirus said.

“And then I heard two more gunshots and it hit the guy next to me and not me.”

As the man fell, Commander Jirus fled.

Rick Mason, a civilian program management analyst, said a gunman was shooting from a fourth floor overlook in the hallway outside his office, America’s ABC reported. He said the gunman was aiming down at people in the building's cafeteria on the first floor.

Mr Mason said that after the shooting, loudspeakers instructed people, who had already been told to find shelter, to exit the building.

Terri Durham, an executive assistance with Navy Systems Command, told CNN she ran into a hallway with a small group only to see a man with a rifle at the far end. He then fired at them, hitting the wall above them as they fled.

“He aimed high and missed,” she said. “He said nothing. As soon as I realised he was shooting, we just said, ‘Get out of the building.’

“We’re lucky he was far enough away [and] he was a bad shot,” she said.

Dewey Carpenter, a Navy employee who worked in building 210, directly across from building 197 where the shooting broke out, told the Washington Post that he and fellow staff had heard a garbled loudspeaker announcement instructing employees in numerous adjoining buildings to “take shelter”. A later email echoed those instructions to stay put in offices and away from windows.

“The mood is solemn, concerned,” he said. “Some people are really scared ... Everyone in here knows somebody next door.”

Obama speaks out

At lunchtime at the White House, just over five kilometres from the scene of the attack, President Barack Obama prefaced a scheduled speech marking the fifth anniversary of the global economic crisis with comments about the shootings.

“I've been briefed by my team on the situation. We still don't know all the facts, but we do know that several people have been shot and some have been killed. So we're looking at yet another mass shooting,”  he said.

“And today it happened on a military installation in our nation's capitol. It's a shooting that targeted military and civilian personnel.

“These are men and women who were going to work, doing their jobs, protecting all of us. They're patriots, and they know the dangers of serving abroad, but today they face the unimaginable violence that they wouldn't have expected here at home.”

Throughout the morning rumours circulated about the number of people injured and killed and there was confusion about the number of gunmen. At press conferences just after midday, Mayor Vincent Gray and Chief Lanier confirmed that one gunman had been killed but that police were looking for two other men seen in the area wearing what appeared to be military-style uniforms.

Security was increased at many of DC’s other sensitive sites, including the Capitol building and the Pentagon.

Shooter arrested before

The Dallas Morning News reported that Alexis was arrested in Fort Worth in Texas on September 5, 2010, for discharging a gun in the Fort Worth city limits.

He is believed to have fired a gun through the ceiling of his apartment into his upstairs neighbour’s apartment, according to NBC5.

His neighbour called the police after the bullet came through her floor and hit the ceiling.

The neighbour told police at the time that she was “terrified” of Alexis, who had previously confronted her about making too much noise, according to The Smoking Gun.

The woman, who was “visibly shaken up”, told police that she believed the shooting was “intentional”.

Alexis claimed at the time that he was cleaning the weapon when it discharged, according to The Smoking Gun. He told police that his hands were slippery and that he had begun "to take the gun apart when his hands slipped and pulled the trigger discharging a round into the ceiling’’.

But Melody McDonald, spokeswoman for the Tarrant County District Attorney, told the Dallas Morning News that Alexis was never charged with a crime in Tarrant County and was released from Fort Worth city jail the same day he was arrested.

"We have to find out what kind of case it was, who it went to. But he never had a pending case in this office," she told the newspaper.

“He has never been charged with a crime in Tarrant County, at least not as an adult.”

Additional reporting by Megan Levy, The Washington Post and wires