A major security operation swung into action at a US military base regularly used by President Barack Obama when someone mistook personnel conducting a routine inspection for a gunman.

Confusion at Joint Base Andrews near central Washington was heightened as commanders were preparing to carry out a drill on dealing with an "active shooter" on the complex at the time.

A distress call reporting a "shooter" sparked a full lockdown of the site, with all personnel urged to seek shelter.

But the lockdown was lifted a short time later for most of the site, and the all clear was given.

No gunman was found and officials said there was no threat to the base or workers.

The base is home to presidential jet Air Force One and other aircraft used by Mr Obama and top members of the US government.

Mr Obama was last at the base on Wednesday night when he returned from a trip to Ottawa, Canada.

Base commander Col Brad Hoagland said in a Facebook post: "We take all threats seriously and reacted to ensure the security of those on the base."

The all clear was given for all areas apart from in the Malcolm Grow medical centre where the "active shooter" report came in. It is not clear why the lockdown remains in place in that area.

Vice-president Joe Biden was scheduled to leave from Andrews on Thursday morning, but his trip was delayed by the alert.

Eyewitnesses said emergency vehicles on the base had on their lights, but no sirens.

At least three people in camouflage and helmets were seen walking working dogs around the three-storey medical building. A number of people were then seen leaving the building, including a person being moved in a wheelchair.