Six workers are reported to have been killed in an explosion at a chemical plant in South Korea on Friday.

Fire officials told the local Yonhap news agency that six employees of a subcontractor died in the accident at a factory owned by Hanwha Chemical Co. A seventh worker was injured.

The explosion occurred when the workers were doing welding to expand a storage tank containing waste. It’s thought that gas trapped inside the tank may have triggered the blast.

“Gas from the waste water, which had piled up inside the tank, was apparently ignited by the welding torch,” one fire official told news agency AFP.

According to a report by CNN, the blast tore out the upper structure of the storage facility, which later collapsed.

About 30 emergency vehicles were sent to the scene, along with around 220 emergency workers, government officials, police and military personnel, the news source reported.

The chief executive of Hanwha Chemical, Kim Chang-bum, offered a public apology for the accident and expressed condolences to the families of the victims. He said that the company would fully cooperate with an investigation to determine the cause of the explosion.

Hanwha Chemical has suspended operations at the plant in the southeastern coastal city of Ulsan. The facility mainly produces polyvinyl chloride (PVC), a general-purpose plastic used in a wide range of products.