A top secret US military aircraft has been spotted in the skies above Seattle, according to bizarre claims online.

A video, posted by an account called ‘Ufo disclosure’ on September 18, shows a triangular UFO speeding across the sky, not far from another aircraft.

The clip, which is captioned ‘Triangle UFO or TR-3B filmed in Seattle’, shows what some claim is a US Air Force spy plane.

TR-3 planes do not officially exist, but according to conspiracy theorists, they are a class of US surveillance aircraft, developed in complete secret.

‘Black Triangle’ UFO sightings have become relatively common in recent years, particularly above the United States, but they have also been ‘spotted’ above Australia and Russia.

It’s thought TR, which it has been suggested stands for ‘Tactical Reconnaissance’, might be to blame for the sightings.

According to some claims online, the class of aircraft was first used during the 1991 Gulf War, to pick out targets for US stealth bombers.

Of course, there is no proof these planes actually exist, but they remains subject to almost relentless Internet speculation, and sightings like this will only add fuel to the fire.