Rescuers pull a person out of the debris of a collapsed workshop after a massive blast ripped through a factory on Monday in Zhangqiu, Shandong province. The explosion killed 13 people. Provided to China Daily

42 workers were in or near the explosives factory at the time; 10 still missing

Thirteen people have died after a massive blast ripped through an explosives factory on Monday morning in Shandong province, authorities said.

Nineteen people were also injured in the accident at 10:45 am inside a workshop at a plant owned by Poly Explosives (Jinan) in Zhangqiu.

Thirty-four workers were in the workshop at the time of the blast, which caused the building to collapse, according to a statement from the city government.

Eight migrant workers were also working near the workshop.

More than 20 firetrucks and three excavators were sent to the scene.

By late Monday, rescuers had pulled 19 workers and 13 bodies from the rubble, the statement said.

The injured workers, including two who were seriously hurt, are being treated at nearby hospitals. Rescuers are still searching for 10 missing workers.

The plant has four production workshops and the one that the explosion destroyed had three floors, said a security guard at the plant who asked to be identified only as Li.

"I was on duty when the blast happened. I saw a small, dark mushroom-shaped cloud rise from the place where the workshop stood after the shockwave of the blast stunned me."

His arms were cut by the broken windows at the security guard office.

Li said he had worked at one of the production workshops for 26 years and suggested the tragedy was possibly caused by a "mechanical malfunction".

"The plant practices a very strict work-safety policy and each worker must pass through four safety checks before being allowed to enter workshops," Li said. "In addition, workers have a strong sense of work safety."

He added that firefighters arrived at the scene two minutes after he called them.

The collapsed workshop produced explosives mainly used in coal mines, he said.

Covering an area of 66 hectares, the plant was a new manufacturing base with an annual output of at least 30,000 metric tons, according to information on the company's website.