At least 25 people have been killed in an explosion at a firecracker factory in the southern Indian state of Tamil Nadu, with four others seriously injured.

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.

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.

A complaint has been filed against the factory owner and foreman, both of whom are absconding, police said, adding that four special teams have been set up to trace and arrest them.

Meanwhile, efforts to identify the victims continued on Monday. District officials said that 22 of the 25 dead had been identified so far - most of them are women.

Some reports say the force of the blast left several bodies charred beyond recognition.

Accidents at fireworks factories in India are not uncommon.

The industry supplies firecrackers for weddings, festivals and other ceremonies and has faced repeated scrutiny over safety standards.

As the investigation continues, questions are being raised about possible safety violations.

Speaking to reporters, district collector Sukhaputra said the factory should not have been operating on a Sunday, calling it a "first violation".

He added that it held a valid licence, but that checks into other possible breaches were under way.

The tragedy also prompted swift responses from politicians.

Prime Minister Narendra Modi described it as "deeply distressing" and offered condolences to the families of those killed.

Tamil Nadu Chief Minister MK Stalin said the deaths had caused "immense sorrow" and that he had instructed officials to ensure all necessary assistance.

Additional reporting by Prabhurao Anandan, BBC Tamil