At least four miners were killed and nine went missing in a colliery gas outburst Friday in China’s Henan province, authorities said.

The outburst occurred at 10.37 a.m. in a tunnel in Ruzhou City, where 250 miners were working underground, Xinhua quoted Li Yongsheng, a mine official as saying.

Four miners were confirmed dead in the evening. Rescuers were searching for the other nine, Li said, adding the incident was being probed.