Washington police say there was no evidence of a shooting at the US capital's Navy Yard, two hours after the base was put in lockdown following reports of an active shooter on the loose.

The spokeswoman said the police investigation was concluded and subsequent inquiries would be done by Navy authorities, who tweeted there was "no sign of shooting".

A caller told authorities shots were heard in Building 197, the same building that was the scene of a rampage by a lone gunman in 2013, in which 12 people were killed.

A Navy officer who was inside the complex told CNN people scrambled for cover after hearing someone shout to get out of the building and stay away from the cafeteria.

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But Lieutenant Commander Scott Williams said he heard no shots or saw any signs of a struggle.

"We heard someone scream, get out of the building, stay away from the cafeteria and we saw everyone running for the exits or adjoining offices," he said.

A fire department spokesman said no injuries or victims were located.

The Navy, which told people at the site to shelter in place, said it was unable to confirm any incident at the base.

Navy spokesman Chris Johnson told media an investigation is underway.

"The entire base is on lockdown," he said.

"Everyone's been ordered to shelter in place, but there's been no confirmed details on what the incident is or may be. We're currently investigating."

Traffic in south-east Washington came to a halt after police and emergency crew arrived at the scene of a reported shooting at the Navy Yard. ( AFP: Mladen Antonov )

Police cars converged on the scene in south-east Washington, and television images showed a huge traffic jam outside the complex.

Authorities stepped up security outside the White House, closing the pedestrian mall that runs along Pennsylvania Avenue and the adjoining Lafayette Square.

Heavily armed police descended on the Navy Yard in Washington after a call to authorities.

The Navy Yard houses offices of the US Navy and is the headquarters for a number of naval operations.

The incident comes with the US on heightened alert ahead of the July 4 national holiday.

The FBI, Department of Homeland Security and National Counterterrorism Centre have warned the public of an increased risk of attack during Independence Day weekend.

The Pentagon ordered new security measures be put in place after the September 2013 shootings by Aaron Alexis, who was killed in a shootout with police.

The Navy Yard, on the Anacostia River, was established in 1799 as a shipbuilding site and a base for defending the city.

The complex houses the Naval Sea Systems Command.

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AFP/Reuters