Fire officials say an explosion at a chemical plant in South Korea has killed five workers. Another person is missing.

The officials say the blast occurred Friday at a waste storage facility belonging to a chemical plant run by Hanwha Chemical Co. in the southeastern city of Ulsan.

The exact cause of the explosion wasn't immediately known. But fire authorities suspect that gas trapped in the storage area might have triggered the blast.

The fire officials in Uslan say the victims were doing welding work when the explosion happened.

The officials spoke on condition of anonymity because they were not authorized to speak to the media about the explosion.