Four workers were killed and two others were missing on Friday when a waste water storage tank exploded at a chemical plant in the South Korean industrial city of Ulsan, authorities said.

The blast at Hanwha Chemical Co. occurred when the workers were doing welding work on the tank, a spokesman from Ulsan Nam Fire Station told AFP.

"Gas from the waste water which had piled up inside the tank was apparently ignited by the welding torch", he said.

An investigation is under way to determine whether the accident was a result of negligence, he added.