Six South Korean workers were killed Friday when a waste water storage tank exploded at a chemical plant in the industrial city of Ulsan.

Six South Korean workers were killed Friday when a waste water storage tank exploded at a chemical plant in the industrial city of Ulsan, a fire official said.

The blast occurred when the workers were carrying out welding work on the tank at the Hanwha Chemical Co. plant, which produces polyvinyl chloride, an Ulsan fire station spokesman said.

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

An investigation was under way to determine whether the blast was caused by negligence.

Four workers were initially found dead at the scene, and rescuers later retrieved the bodies of two others while sifting through the mixture of debris and toxic waste water.