WASHINGTON (AFP) - US Joint Base Andrews outside Washington, the home base of the presidential plane Air Force One, said on Thursday (June 30) that a "miscommunication" about an active shooter training exercise led to a brief lockdown of the site.

The Maryland military base explained via Twitter that there had been plans for a "no-notice active shooter exercise in the late morning."

"However, reports of a real-world active shooter situation at the medical facility were miscommunicated before the exercise was able to begin," it said.

"There was a misidentification of the security forces emergency services team who were conducting a routine inspection of the facility," it added.

The base, home to thousands of military personnel and their families, was on lockdown for about an hour, with everyone ordered to shelter in place.

"It appears to have been a mistake, and we'd like to reduce the number of mistakes made in this way, no question about it," Pentagon chief Ashton Carter told reporters.

Someone apparently called the emergency number 911 when they saw the drill, The Washington Post reported, citing an unnamed federal law enforcement official.

The lockdown delayed the travel of Vice-President Joe Biden, who was due to head to Ohio for a campaign event, according to his office.

US President Barack Obama regularly plays golf on the base.