The explosion happened about 10:45 a.m. inside a workshop of the Poly Explosives Co. plant in Zhangqiu City, China's Xinhua News Agency reported.
A release from the publicity department of the Zhangqiu City government said 34 workers were in the workshop when the explosion happened and eight other workers were nearby.
So far, 31 workers have been pulled from the wreckage, including the 12 who were killed, the statement said.
Other workers were injured in the blast, including two who sustained serious injuries, and were transported to local hospitals, officials said.
Rescue crews were still working to locate the 11 missing workers.