A gas blast killed 13 coal miners and injured nine in China's northern Shanxi province, Xinhua news agency reported on Saturday.

The explosion took place on Friday at Niuxinhui Coal Mine in Ningwu county of Shanxi province, when 30 miners were working underground, Xinhua said, citing Tan Shengwang, head of the county's work safety administration.

The accident killed 11 miners on the spot. Two others, badly injured, died on the way to hospital, while nine are still in medical care.

The legally-registered coal mine was undertaking an expansion project approved by the provincial coal industry authorities, Xinhua said.

The cause of accident is under investigation.

Deaths in Chinese coal mines, the world's deadliest, fell by 20 percent in 2006 while output rose by about 200 million tonnes.

An average 13 coal miners a day, or a total of 4,746 people, died in thousands of blasts, floods and other accidents last year.