Sun Huashan, deputy director of the State Administration for Work Safety, told state television that a crew drilling in a natural gas field had misjudged the amount of gas in the well which burst.

He also said that after deadly fumes began to flow from the well, the team failed to ignite them, which would have prevented them from spreading.

"This was an accident due to negligence. Those people who were responsible will be dealt with," Mr Sun said.

A total of 64,000 people fled after the natural gas well burst in the town of Gaoqiao near Chongqing, releasing a high concentration of natural gas and sulphurated hydrogen.

Thousands of those displaced began returning to their homes this week.

Survivors described the gas as an acrid black fog that clung to the ground, choking them and burning their eyes and skin.

The gas field was operated by the state-run oil company China National Petroleum Corporation.