Reports have said an "active gunman" has fired shots at the Navy Yard in Washington DC, site of a mass shooting in 2013.

An earlier report had claimed that the shooting was a hoax, but this related to a separate incident at a mosque in New Jersey.

Video footage showed armed US Marshals wearing body armour outside the gates of the defence facility on the Anacostia River after it was placed on full lockdown following reports of shots being fired.

A large number of police vehicles were at the scene, with the gates to the yard shut and no one allowed to enter.

Navy spokesman Petty Officer 1st Class Pedro Rodriguez said shots were reported in a building on the campus, triggering the full lockdown.

US media reports had said that the alert is focused on building 197 on the base. In September 2013, military contractor Aaron Alexis killed 12 civilian workers after he went on a shooting rampage at building 197, before he himself was shot dead.

A tweet from the US Navy account said: "Navy Yard on lockdown/shelter in place. No incident can be confirmed as of yet. More to follow."

Reporter Sam Kiley said: "They will be scouring the naval yard and trying to establish the whereabouts of the gunman."

"It's just very much at the early stages of what is turning out to be a massive security operation."

"If this is part of a wider plot that will also be a concern, but they have learned in the US not to jump to conclusions."

The pedestrian plaza on Pennsylvania Avenue, in front of the White House, was sealed off by law enforcement after the Navy Yard alert.

The Navy Yard, in the southeast of Washington DC, is the country's oldest naval installation.

It comes amid heightened security in the US ahead of US Independence Day celebrations on Saturday.