A passenger jet has been forced to make an emergency landing, after an unidentified “foreign object” smashed into its nose cone at 26,000 feet.

A large dent and scraped paintwork was visible on the Air China Boeing 757 after it landed safely at Chengdu Airport, having been forced to turn back after the pilot said he was struggling to control the plane.

Investigators are said to be baffled by the dent, reportedly ruling out a bird strike as no blood or feathers were seen around the impact site, but insisting traces of whatever caused the damage are likely to be found under closer inspection.

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The plane was flying at around 26,000 feet, climbing over China 20 minutes after take-off during an internal flight between Chengdu and Guangzhou, when passengers are said to have heard a loud bang.

At the time the pilot is thought to have been unaware of the damage, but contacted air traffic control to report stability issues shortly after hearing the noise.

He was instructed to return to Chengdu, where the damaged nosecone was seen and photographed by passengers disembarking the aircraft.

The images were shared on Weibo - the Chinese version of Twitter – where users began discussing conspiracy theories about the accident.

The real cause of the impact may never be known, however, as Air China has not confirmed if it will make any investigation results public.