A gas explosion at a coal mine in central China has killed 29 workers, Chinese authorities said on Sunday.

Six other miners survived Saturday evening's blast at a state-owned coal mine in Hengyang city in Hunan province, China's State Administration of Work Safety said in a statement on its website.

Five of the workers were rescued, while one climbed out of an air shaft of the Xialiuchong Coal Mine, the statement said.

The work safety administration said rescue work was complete as no other miners were working at the time of the explosion, which happened 250 metres underground.

China's state television CCTV showed relatives of the workers waiting for news outside the coal mine.

The chief of the provincial coal mine safety bureau told CCTV that the explosion was probably the result of a large outburst of gas being ignited by sparks produced by machines.

CCTV reported that the mine's operating licence had been revoked in the first half of this year because it did not adopt measures to pump out dangerous gases from underground, but that the mine continued production without permission.

China's coal mines are the most dangerous in the world.

Demand for coal induces many producers to sidestep safety regulations, although conditions have improved and a number of small, illegal mines have been shut.

Annual fatalities are now about one-third of the high of nearly 7,000 in 2002.