JIAOZUO, Henan - Seven coal miners have been confirmed dead and 11 others are still missing after a sudden release of pressurized gas sent tons of coal flying and blocked the mine shaft in Jiaozuo of Central China's Henan province, the local safety watchdog said on Thursday.

The sudden release of gas - called an outburst - occurred at 12:36 am on Thursday at Jiulishan Coal Mine when 18 miners were working underground. The 18 workers entered the pit just half an hour before the accident, according to the local government publicity office.

Rescuers had found the bodies of seven miners as of Thursday night and the location of the 11 missing workers cannot be determined, Zhang Feng, chief engineer of the rescue team, told China Daily.

Two rescue teams, each consisting of nine rescuers, were dispatched into the pit while a third team was standing by.

Two piping machines were operating at full capacity to pipe gas from the accident scene.

According to Zhang, the largest obstacles to the rescue work are the high density of gas and the large amount of coal that needs to be cleared from the shaft.

Rescue teams have started to remove coal from the pit to search for the missing miners as the gas is being piped out.

Experts estimate that there are 1,500 tons of coal needing to be removed, CCTV reported.

The coal mine is legally licensed and has been in operation since 1983. It has a yearly production of 1 million tons.

According to the local coal mine safety watchdog, this is the most severe coal mine accident this year in Henan.

The Jiulishan Coal Mine belongs to Henan Coal Chemical Industry Group Co Ltd, a State-owned company listed among the Fortune 500.

Two other gas explosions in two coal mines in Northwest China's Shaanxi province on Aug 16 and in Chongqing municipality on Aug 17 killed 24 people.