The impact of the blast, which occurred near Shijiazhuang city, capital of Hebei province, was felt in at least three nearby villages, the official Xinhua news agency said.
Photographs posted on the Internet purportedly of the blast site showed a large crater surrounded by debris. The walls of several workshops that surrounded the site had been destroyed.
The cause of the blast is still under investigation and rescue efforts are ongoing, the State Administration of Work Safety said on its website.
The casualty toll came from a preliminary report, the administration said, indicating it could rise.
Safety standards are regularly flouted in China, and workplace accidents remain common despite repeated pledges by the government to improve regulations and oversight.
Nearly 50,000 people died in work-related accidents in the first nine months of 2011, the administration said earlier.
Last week, 13 workers were killed at an explosion at a northeast China steel factory, while three people died in a gas leak at a steel plant in eastern China's Nanjing city, state press reports said.
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