SEOUL - Four workers have been identified dead, with one missing, in an explosion accident near a waste water disposal facility of a Hanwha chemical factory in the country's southeastern city of Ulsan, YTN and Yonhap News Agency reported Friday citing firefighting authorities.

The explosion occurred at about 9:19 am (0019 GMT) at the second plant in Ulsan of the chemical unit of South Korean conglomerate Hanwha Group.

Four workers, who were welding the upper part of the waste water pool, were killed with the explosion, and one has been missing.

Residual gas was estimated to have been exploded during the welding operation, but the exact cause is under investigation.