Pentagon puzzled by mystery drones swarming military base

They were flying at around 100mph at 3,000 to 4,000 feet with witnesses saying they sounded like a parade of lawnmowers.

The incursion triggered two weeks of meetings at the White House involving officials from the Pentagon, including staff from its UFO office and the FBI.

Under federal law, the military is only allowed to shoot down drones over military bases if they pose a direct threat.

If they are suspected of snooping, although that is illegal, it does not mean they can be brought down, and members of Congress have called for powers to be strengthened.

In February last year, China triggered a security alert after a suspected surveillance balloon was shot down by the US Air Force.

Experts at the time believed the balloon, which flew over a major US missile silo, was a show of force rather than a serious attempt to gather information.

The latest reports are not the first time the alarm has been raised in the West by the use of drones.

Two years ago, Norway sent warships and fighter jets to patrol offshore oil platforms after drones, believed to be from Russia, were seen buzzing overhead.

Last month, Lloyd Austin, the US defence secretary, announced an expansion of its Replicator programme which has been developing attack drones to be used against China.

The drones will also be used to defend US bases at home and abroad to fight off attacks by explosive-laden drones from Iran-linked groups, which have been responsible for the deaths of several American soldiers in the Middle East.