As many as 20 people were killed in an explosion at a firework factory north of Bangkok, the capital of Thailand, reports said.

According to preliminary reports, authorities inspected the site but were yet to find the survivors. Reports said that it was not immediately clear what caused the incident, which took place mid-afternoon in Suphan Buri province about 120 km (74.56 miles) north of Bangkok, police said.

Police reports said that several were injured in the incident.

Prime Minister Srettha Thavisin was briefed on the explosion while at the World Economic Forum in Davos, Switzerland, and urged a swift investigation.

“The prime minister ordered an inspection of the plant to see if it was operating legally and if the explosion was caused by negligence. The law must be enforced to the fullest extent because there are innocent deaths and injuries,” Srettha’s office said.