"There was an explosion and in the impact the building collapsed and five workers were trapped in the concrete rubble and they were dead," a senior Villupuram District Fire and Rescue Services official told PTI.
Eleven workers were injured, he said.
"The injured have been rushed to hospital," he said adding of the wounded, six were women and five men.
"All the dead were men and work is on to clear the remaining concrete rubble," he added.
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"It was a small building with three rooms. While one room was used for making firecrackers, other rooms were used for storing the finished products," he said.
On the reason for the explosion, he said it is being ascertained.
Following the mishap, top district, police and fire officials visited the spot and held inquiries.
Police said they have begun a probe.