Fears were growing on Friday for 23 workers trapped underground after a blast at an illegal coal mine in China that killed 20 of their colleagues, the latest disaster to hit the industry in the country.

Hundreds of rescuers and medics have rushed to the site in southwest China to try to save the missing miners, but their efforts were being hampered by a gas leak, official media said, as distraught relatives looked on.

Ventilation machines were pumping methane gas out of the shaft while rescuers took turns heading down the pit to search for survivors, the Xinhua news agency reported.

"The rescue is difficult because the gas levels remain high and may lead to an explosion at any time," Tan Xiaopeng, a fire control official in charge of the rescue, told Xinhua.

The mine was hit Thursday by a "coal and gas outburst" -- a sudden and violent ejection of coal, gas and rock from a coal face, which can cause serious injuries and damage machinery, a local mine safety official told AFP.

China News Service (CNS) and Xinhua said 20 people had been confirmed dead at the remote Sizhuang Coal Mine in Shizong county, in the province of Yunnan, while 23 others were trapped.

Police had cordoned off the scene as relatives of the missing held a tearful vigil.

Quoting local work safety officials, Xinhua said the mine was operating without a licence after its permit was revoked a year ago.

The accident comes days after a rock blast in a coal mine in the central province of Henan trapped dozens of workers underground.

Most were eventually pulled out after a 40-hour rescue operation, though 10 were killed.

Coal mine accidents are common in China, where work safety is often neglected by bosses seeking a quick profit.

Last year, 2,433 people died in coal mining accidents in the country, according to official statistics -- a rate of more than six workers per day.

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

China's rapid economic growth has caused demand for energy, including coal, to surge.

The Asian nation is the world's leading consumer of coal, relying on it for 70 percent of its growing energy needs.

Over the past eight years it has on average built one coal-fired power station a week. And with the arrival of winter, mines are operating at full capacity.

Fatalities at Chinese coal mines peaked in 2002 when 6,995 deaths were recorded, sparking efforts by the government to boost safety standards.

In its latest campaign, the government last year issued a policy that required six kinds of safety systems -- including rescue facilities -- to be installed in all coal mines within three years.

But accidents still occur on a regular basis. Last month, a gas explosion at a state-owned coal mine in the central province of Hunan left 29 miners dead.