A rescuer prepares to enter a pit in an effort to find miners who were trapped after a gas leak at at a coal mine near the city of Qujing, Yunnan province, Thursday Nov 10, 2011. As of 3:40 pm Thursday, 23 people remain trapped. [Photo/Xinhua]

QUJING, Yunnan - Killing 20 miners and trapping another 23, a powerful gas burst in an illegally-operated coal mine in southwest China on Thursday brought about the second deadly Chinese coal mine disaster in a week, raising the alarm for mining safety in the country.

Ventilation machines are now pumping excessive methane gas out from the shaft while hundreds of rescuers take turns to descend into the pit to find the trapped miners at Sizhuang Coal Mine near the city of Qujing, Yunnan province, according to Xinhua reporters who reached the scene over  rugged roads by Thursday afternoon, ten hours after the accident.

Coal dust that was forced out of the shaft by the powerful burst covered the ground for 10 meters around the pit.

Local work safety officials said the private mine, with an annual output capacity of 90,000 tonnes, was operating without a license when the accident occurred. The mine's license was revoked a year ago.

A preliminary investigation showed that the gas leak occurred at one platform inside the shaft but gas later spread to another platform, trapping a total of 43 miners in the two areas at about 6:30 am Thursday.

Instead of "gas leak," the Qujing municipal government said earlier that a "gas explosion" had ripped through the mine.

A handful of tired-looking, dust-covered firemen coming out from the shaft told Xinhua that they descended into the pit only to find 20 bodies.

Rescuers took life detectors and toxic air monitors with them. The area was cordoned off by police while 30 ambulances parked inside.

Women sat on the ground wailing while other relatives of the miners  paced anxiously nearby.

Luo Lin, director of the State Administration of Work Safety, ordered local authorities to do anything possible to save the trapped miners.

Zhao Tiechui, deputy director of the State Administration of Work Safety, and Peng Jianxun, deputy director of the State Administration of Coal Mine Safety, are en route to Qujing to oversee rescue efforts.

The mining industry has been a pillar of Shizong's economy, making up 65 percent of the county's industrial output and contributing 54 percent to its GDP last year.

The accident was the second deadly disaster to hit China's dangerous mining sector in a week. Last Thursday, a rock collapse occurred in a coal mine in central Henan province, triggered by a 2.9-magnitude earthquake. Eight miners were killed but 53 others, including 45 who had been trapped underground for 40 hours, were saved in a miracle rescue.

Rescuers working at Sizhuang said they hoped the miracle could be repeated.

China relies heavily on coal to power its massive economy. Lured by huge energy demands, mine bosses have often been found forcing workers to labor in conditions defying safety rules ordered by the government.

Gas explosions, floods and fires have been frequent occurrences in China's coal mines, killing hundreds, if not thousands each year. Official data shows that more than 2,600 people died in mining accidents in 2009, despite a steady decline in mining deaths over the past few years amid strengthened safety measures and the closure of many illegal mines.

Chinese Premier Wen Jiabao once ordered mine managers to spend some time in the shafts along with their workers in order to raise their awareness of work safety.