A gas explosion at a coal mine operating without a proper licence has killed 29 miners in central China, state media said Sunday, the latest in a series of deadly accidents to hit the industry.

Some 35 miners were working in the state-owned Xialiuchong Coal Mine in Hengyang city, Hunan province, at the time of the blast, which happened early Saturday evening, the official Xinhua news agency said.

The bodies of all the dead had been recovered. Six miners had been rescued and were being treated in hospital to the relief of relatives, who had gathered outside the mine awaiting news.

The mine, which was owned by the city government, was legally registered, but the provincial government suspended its production licence earlier this year for lack of adequate safety measures, state television said.

The explosion occurred after sparks from machinery ignited flammable gas that had filled the mine, it said.

An official of Hunan's mine safety bureau, reached by AFP, declined to comment. "I can't tell you anything, because work is still on-going."

The 40-year-old mine had more than 500 employees. Some 160 workers were on shift at the time of the accident, but only 35 were actually underground, state television said.

More than 40 people were involved in the rescue and recovery operation. The head of the central government's work safety agency, Luo Lin, and Hunan governor Xu Shousheng had travelled to the scene to direct the efforts.

China's mining industry has a notoriously poor safety record with its mines considered among the most deadly in the world.

Earlier in October, a gas explosion in a coal mine in China's southwestern city of Chongqing killed thirteen people, while another blast in the northern province of Shaanxi killed 11 miners.

In 2010, 2,433 people died in coal mine accidents in China, according to official statistics -- a rate of more than six workers per day.

Labour rights groups say the actual death toll is likely much higher, partly due to under-reporting of accidents as mine bosses seek to limit their economic losses and avoid punishment.

As China's rapid economic growth has caused demand for energy, including coal, to surge, some mining bosses have put the safety of workers at risk to chase profits.