Water burst into the small privately-owned mine near Lu-liang late on Sunday night. Rescuers have been pumping water out of the pit. Local authorities said it was impossible to say whether the workers were still alive.
Water had accumulated following several weeks of heavy rains, which have affected regions all over China in recent weeks.
The municipality of Chongqing in southwestern China experienced its largest downpours for several years in mid-July. Ensuing floods, landslides and mud-rock flows have killed 42 people in the city over the past week, the agency said.