Sixteen people were killed when two tonnes of dynamite were detonated at a coal mine in north China, creating a much larger explosion than workers expected, the government and state media said Friday.

The accident, which happened Thursday evening at a mine in the northern Ningxia Hui region, also injured 46 people, including 12 seriously, according to China's work safety bureau.

The 200-metre (660-foot) safety zone established by workers outside the blast site proved inadequate when the explosion threw stones and rocks more than a kilometre (0.6 miles), the official Xinhua news agency said.

The explosion was so powerful that it not only killed six workers inside the shaft, but also five people driving near the mine entrance, according to Xinhua.

A rescue operation was still under way Friday, as officials tried to discover if more people might be missing from the blast.

Police held two people from a company in charge of the blasting operation for questioning, Xinhua said.

Early this month, China issued new rules ordering mines to store their explosives in a safer manner.

Blasts often happen in Chinese mines, but they are more frequently caused by gas accumulating underground than by explosives mishandled by the miners.

Nearly 3,800 people died in Chinese coal mines last year, according to official figures, although independent monitors say the real figure is probably much higher as many accidents are covered up.