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The baffling clip – filmed over Spain – begins with the mysterious object coming into shot directly below the aircraft.

It appears to be diamond-shaped as it travels through the skies.

“What the f***?” the stunned cameraman says, before zooming in further.

The craft looks completely flat but almost as soon as it comes into view, it disappears into the distance.

(Image: YOUTUBE/GETTY)

More than 35,000 people have viewed the startling video since it was posted to YouTube on Wednesday (April 5).

“I’ve witnessed a UFO from my airplane window seat before so I DO believe this…trust me it’s real,” one user commented.

Another wrote: “Amazing! It seems to be moving slow at first then speeds up.”

Some users dismissed the video, claiming it had been edited.

(Image: YOUTUBE)

But one viewer suggested: “The tricky part of adding an image from a camera phone is the perspective and speed…this passed the test of enlargement.”

Others believed the craft was in fact a weather balloon.

A startlingly similar craft was filmed recently hurtling over rush hour traffic in Mexico.