Police have detained five coal mine executives after 12 people died when a gas pocket ignited in a coal mine in north China's Shanxi Province, authorities said Monday.

The accident occurred at 11:30 p.m. Sunday when 16 miners were working in a shaft in the Donggou Colliery Co., Ltd. in Jiexiu City, said a spokesman with the Shanxi Provincial Work Safety Administration.

The police did not reveal the identities of the detainees.

The mine was expanding its annual production capacity from the authorized 30 tonnes. The workers violated a safety rule that prohibited the demolition of a wall between the shaft and a disused area of the mine where the gas was accumulated in high density, the spokesman said.

The four survivors were under observation in hospital. Doctors said they were stable.

The Jiexiu city government has launched safety inspections to prevent more accidents.