(CNN) An explosion at a chemical factory in the South Korean port city of Ulsan has killed four people, an official at the city's fire department has told CNN. Two people remain missing following the incident.

Fire department officials initially heard the blast at the Hanwha Chemical plant and were trying to determine its location when the emergency call came in.

Initial indications suggest the explosion, which occurred at around 9 a.m. local time Friday (8 p.m. Eastern, Thursday), was caused while workers were welding the interior of a waste water storage facility.

The blast tore out the upper structure of the storage facility, which subsequently collapsed.

Almost 30 emergency vehicles were dispatched to the scene, along with around 220 emergency workers, government officials police and military personnel.

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