The condition of those underground isn't known and rescue work is continuing, said the official, reached by phone at the Yongzhou district government propaganda office in the sprawling Chongqing region.

He refused to give his name, as is common with Chinese public servants.

The blast occurred shortly before noon, state media reported. Such accidents are often caused when a flame or electrical spark ignites gas leaking from the coal seam.

Ventilation systems are supposed to prevent gas from becoming trapped.

China's mining industry has long been among the world's deadliest, although safety improvements and the closure of smaller, more dangerous mines have reduced the death toll in recent years.

China is the world's largest producer and consumer of coal but has announced plans to shutter more than 1,000 outdated mines.

The head of China's State Administration of Work Safety said this year that struggling coal mines might be likely to overlook maintenance.