The building, which fell Monday morning, is near the village of Houshan in Hebei province, China's state-run Xinhua News Agency reported.
The workers, employed by the Fengshanbao Iron Mine, were sleeping in the building at the time of the collapse, local authorities said.
Rescuers said complicated geological conditions and the structure's narrow foundation were making rescue work more complicated.
The cause of the collapse was under investigation.
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