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Footage showing a heat-emitting UFO which was invisible to the human eye has been captured by a police helicopter as it was flying over Bristol.

A dark, circular shape is seen hovering in the sky as police monitors can be heard observing what they are seeing - although they have admitted they have no idea what it was.

The clip was filmed on Saturday September 17 and shared online by NPAS St Athan , the National Police Air Service based in South Wales.

They explained to their Twitter followers that the footage, which says it was 8.30pm, was actually taken about 9.30pm when it was pitch dark.

(Image: NPAS St Athan/Twitter)

They posted the original clip on September 23, writing: "Any suggestion?? Nothing seen by local ATC," referring to Air Traffic Control. They added an alien emoji to the tweet.

The clip, which they have revealed is part of a much longer video, was filmed "over the Bristol channel at approx 1,000ft", they explained.

(Image: NPAS St Athan/Twitter)

The filming was set to 'black hot', which means there is heat coming off the craft.

"Really difficult to judge the size but we filmed it for just over seven minutes as we were coming back into land," they add.

(Image: NPAS St Athan/Twitter)

Although some have stated it's obviously a balloon or lantern, they say that it's not so easily explained.

"We really don't know but doubt it was a lantern as it was going into the wind."

Craft expert UFOs-Exposed shared the footage with their followers, stating that "the pilot [says] the object wasn't visible in daytime mode, only the camera's infrared mode."