Chinese authorities said preliminary investigations indicate the explosion, that occurred at 7 a.m. on Tuesday, was caused by illegal mining activities in a mine in Shizuishan. Twenty miners were working, Efe news agency reported.
The officials of the company that owned the mine have been arrested, the agency added.
Deputy Mayor Wu Yuguo said at a press conference on Wednesday that the excessive concentration of gas and debris was hampering rescue efforts.
Poor safety conditions in mines in China claimed 171 lives in accidents in 45 coal mines last year, according to the country's National Energy Administration.
Battling excess capacity in the coal and steel sectors, China has proposed to shut down over 1,000 obsolete coal mines with a total production capacity of 60 million tonnes.
