Four workers were killed and four others injured after an explosion at a petrochemical factory in the southern city of Yeosu in South Korea, Yonhap news agency reported Friday. 

The explosion occurred at 9:26 a.m. in Yeosu with eight people believed to be present at the scene, Yonhap said.

Police and firefighters said they believe the incident happened during testing of the factory's heat exchange system, the report said.