The disaster occurred on Friday in the northern city of Dengfeng, in Henan province, the official Xinhua News Agency said, citing the Henan Provincial Coal Industry Bureau.
A total of 56 miners were working in the Malingshan Coal Mine at the time of the accident, and 41 escaped, Xinhua said.
The gas was released while miners were drilling underground to test a coal seam, Xinhua said.
A 37-member rescue team was searching for the three missing miners, Xinhua said. It said rescuers were spraying water inside the mine to minimize airborne coal dust.
The mine recently expanded its production capacity from 60 000 tons to 300 000 tons of coal per year, Xinhua said, though it didn't say whether the accident was believed to be connected to that expansion.
China's coal mines are the world's deadliest, with more than 5 000 fatalities a year in fires, gas leaks and other disasters that often are blamed on indifference to safety rules or lack of required equipment.Last Mod: 20 Eylül 2018, 18:16