Air Force chiefs have been left completely stumped by video of a UFO dropping "fireballs" near a military installation.

The footage was recorded in the US city of Mesa, Arizona, not far from a site, codenamed Outlaw, used for military operations.

But the Federal Aviation Administration, Luke Air Force Base and the Army National Guard could offer no answers when approached with the clip.

Kerri Burnett and her fiancé, Doug Maier, who recorded the footage, were watching the planes coming in and out of the nearby Sky Harbor airport when the unusual lights caught their attention.

To read more and view the video, click here.