BANGKOK (AP) — An explosion at a fireworks factory in central Thailand killed at least 20 people on Wednesday, the government’s disaster relief agency said.

People were also injured in the incident, the Department of Disaster Prevention and Mitigation said, but it did not say how many. Officials were working to secure the site in Suphan Buri province and help people who were impacted, it said. The cause of the explosion was being investigated.

Suphan Buri is about 95 kilometres (60 miles) northwest of Bangkok, in the heart of Thailand’s central rice-growing region.

The office of Prime Minister Srettha Thavisin, who is in Switzerland for the World Economic Forum, distributed a video showing him being told over the phone by the regional police commander that there were 20 to 30 workers at the factory at the time of the explosion and that none of them could be found.

Kritsada Manee-In, a rescue worker with the Samerkun Suphan Buri Rescue Foundation, who earlier estimated that around 15 to 17 people had been killed, said an exact count was difficult because the bodies were in pieces.

Photos posted on social media showed a thick plume of black smoke rising over the scene of the blast in an otherwise unoccupied rice field. Photos posted online by local rescue workers showed the factory site virtually levelled flat aside from debris and body parts.

National police chief Torsak Sukvimol confirmed local news reports that there had been another explosion at the factory in November last year that killed one worker and seriously injured three others.

He said police would pursue legal action for any wrongdoing involved.

The blast, whose cause was not immediately clear, came less than a month before Chinese New Year in February, when demand for fireworks is strong.

In July last year, a large explosion at a fireworks warehouse in southern Thailand killed at least 10 people and wounded more than 100, according to officials.

That explosion in Narathiwat province was in a residential area and damaged about 100 houses within a 500-metre (1,640-foot) radius, according to the Department of Disaster Prevention and Mitigation.

The Narathiwat governor said that the blast was likely ignited by construction work in the warehouse, with sparks from metal welding causing the fireworks stored inside to catch fire and explode.