An explosion on an oil tanker under construction at an STX Offshore & Shipbuilding plant in a southeastern province of South Korea has left four workers dead, local media reported Sunday, citing national fire agency.

Kanal24 reports that the blast occurred at 11:37 a.m. local time (02:37 GMT) at the plant in Changwon, South Gyeongsang Province.

The workers were inside the oil tanker when the accident took place.

The reason behind the explosion is as yet unkown.