The blast occurred at the Vanaja Fireworks Industry in Virudhunagar on Sunday evening. The factory was supposed to be closed but about 50 workers were inside at the time, officials said. BBC reported.
A second explosion occurred while rescue operations were on, injuring 13 more people, though none sustained serious burns, District Collector NO Sukhaputra told ANI news agency.
The cause of the blast is not yet known and an investigation is under way.
An eyewitness, who is being treated for injuries, told BBC Tamil that around 25 people were working in the veranda in front of the factory when the fire broke out, damaging at least three rooms and reducing parts of the building to rubble.
Rescue operations - which have now concluded - were then significantly slowed down as firecrackers continued to explode inside the building.
