Experts in aviation and avid UFO conspirators have been left baffled by photographs that appear to show a mysterious triangular-shaped flying object flying over US airspace.

The photos, captured by Steve Douglass and Dean Muskett, show what appears to be an aircraft flying across on Amarillo International Airport 10 March, with another contrail next to it.

Experts say the object’s shape is unlike any known plane, which was seen accompanied by two other aircraft.

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The images have prompted speculation that the aircraft could be a classified military craft.

In his blog, Douglass said that after reviewing images, they noticed that the aircraft had almost a boomerang shape, meaning it could not be a B-2 stealth bomber aircraft as previously thought.

Bill Sweetman of Aviation Week said: "Three of us here - myself, Graham Warwick and Guy Norris - concur that the photos show something real.

“The photos tell us more about what the mysterious stranger isn't than what it is. The size is very hard to determine, for example, although the image size at contrailing height suggests that it is bigger than an X-47B.

“However, the basic shape - while it resembles Boeing's Blended Wing Body studies or the Swift Killer Bee/Northrop Grumman Bat unmanned air system - is different from anything known to have flown at full size, lacking the notched trailing edge of Northrop Grumman's full-size designs.”

Douglass believes they may have picked up voice traffic from the craft as well, which would suggest it was manned instead of remotely controlled, but nobody has been able to identify the mystery craft so far.