BEIJING (Reuters) - Four workers were killed and nine were missing after an explosion at a small coal mine in China’s Guizhou province on Monday night, state-owned Xinhua News reported on Tuesday citing local authorities.

The incident happened at Zimujia coal mine, run by private-owned Pannan Coal Investment Co, in Panzhou city. The coal mine has designed annual capacity of 300,000 tonnes.

Cause of explosion was unclear, while rescue work was underway, it said.

Benchmark thermal coal futures soared as much as 5.5 percent to 622 yuan ($90.80) a tonne, their highest in 11 months, during early trade on Tuesday, as market expects more stringent inspections on coal mines across the country might lead to tight supply.

The accident comes not long after a blast at an iron ore mining project in Liaoning that killed 11 people in June.

According to Guizhou Energy Administration file, the coal mine was ordered to rectify on some safety issues during a safety inspection conducted by local authorities in late April.

Pannan Coal cannot be reached for comment. Panzhou city government declined to comment.