ZHENGZHOU - Five people died and seven others were left trapped after a colliery gas outburst in Central China's Henan province Wednesday.

Rescue work is ongoing at Xingyu colliery in the city of Dengfeng, though the location of those trapped remains unclear as debris blocking the shaft has not been completely cleared.

The accident at 5:35 pm Wednesday saw 39 miners escape to safety.

Gas density in the colliery was lowered to safe levels Thursday afternoon, making it possible for rescuers to enter the shaft.

"The high gas density and huge amount of coal debris in the shaft affected our rescue work at the beginning," said Hu Wenliang, head of a rescue team.

"We initially judge the accident to be caused by the improper operations of on-duty workers. Temporary supporting measures were fully implemented when miners were doing maintenance work," said Bu Lianying, chief engineer of the colliery.

The Xingyu colliery, built in 1975, is licensed and has an approved annual capacity of 330,000 tonnes.

The colliery suspended production for an overhaul on Dec 12 and resumed operation on Dec 29 with government approval, according to Chen Zhilong, vice mayor of Dengfeng.

An investigation is underway.