Rescue efforts are underway to search for 23 miners trapped by a gas leak that killed 20 others at an illegally operated coal mine in southwest China.

China's Xinhua news agency reported Thursday that ventilation machines were pumping methane gas out of a shaft, while hundreds of rescuers took turns descending into the mine to search for survivors.

A preliminary investigation found that the gas leak at the mine in Yunnan province occurred early Thursday, trapping 43 miners. Rescue workers later exiting the shaft told Xinhua they found 20 bodies.

Local officials say the private mine was operating without a license when the gas leak occurred.

The accident came a week after eight miners died in an explosion at a coal mine in the Chinese city of Sanmenxia. The blast trapped 45 miners who were later rescued.

And last month a gas explosion at a coal mine in Hunan province killed 29 workers.

China's coal mines are the deadliest in the world because of poor compliance with safety standards. In 2010, China reported that 2,433 people died in coal mining accidents. That number is an improvement over recent years.