Four South Korean workers were killed in an explosion at a shipyard in the country\'s southeastern industrial city of Ulsan on Friday. The four workers were working in the cabin of a 4,200-ton vessel when the explosion occurred at about 9 a.m. local time, Yonhap news agency reported. Police said the explosion at Sejin Heavy Industries, a ship components maker, is believed to have been caused by sparks from an oxygen-fuelled cutting machine, according to Yonhap.