ULSAN (Yonhap) -- The head of Hanwha Chemical Co.
offered a public apology Friday over a deadly explosion at a plant that killed six workers and injured one.

Kim Chang-bum also expressed condolences to the families of the victims -- the six workers of a subcontractor.

The blast occurred at around 9:19 a.m. when the workers were doing welding work on a storage tank containing waste at the Hanwha's plant in the southeastern coastal city of Ulsan, fire officials said.

The officials suspect that a welding spark came in contact with a heat source, mainly methane and biochemical gas from the storage, triggering the blast.

Kim vowed to fully cooperate with an investigation to determine the cause of the accident as he suspended the operation of the plant.

The Ulsan plant mainly produces polyvinyl chloride, a plastic used in a wide range of products. It is the second-biggest chemical factory operated by Hanwha.

Hanwha Chemical is one of the units of Hanwha Group, South Korea's ninth-largest family-controlled conglomerate, whose businesses also include insurance, energy and defense. (Yonhap)