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China's first astronaut says he heard mysterious knocks during his first flight in space - but no one has been able to explain the cause of them.

Astronaut Yang Liwei said the strange noises left him feeling very nervous and he looked out into the vast emptiness of space but couldn't see anything out of the ordinary.

Mr Yang, now a major general in China's Air Force, said it sounded like someone was knocking the body of the spaceship, comparing the noise to that of a hammer hitting an iron bucket.

The 51-year-old's unexplained experience has raised all kinds of theories, including aliens , and he said he has never heard the sound again since returning to earth.

(Image: China.org.cn)

In 2003 Mr Yang became the first person sent into space by the Chinese space programme, orbiting earth several times during a 21-hour flight.

He recalled the experience in a recent interview, telling Chinese media the mysterious knocking sounds occurred without rhyme or reason.

He said: "A non-causal situation I have met in space is a knock that appeared from time to time.

"It neither came from outside nor inside the spaceship, but sounded like someone is knocking the body of the spaceship just as knocking an iron bucket with a wooden hammer.

Mr Yang revealed he was very nervous when he heard the sounds and moved close to the spaceship's porthole to investigate the cause.

He failed to see anything out of the ordinary.

When he returned to earth he explained the noises to officials with the space programme and tried to imitate it with instruments.

The sound has also been heard by Chinese astronauts who have since been blasted into space.

Mr Yang said: "Before entering space I have told them that the sound is a normal phenomenon, so there is no need to worry."