Four workers were killed and 12 injured in a boiler explosion on a cargo ship in the eastern Chinese city of Nanjing Friday night, a shipyard said Saturday.

The explosion occurred at about 11 p.m. Friday on a bulk carrier as the Zijinshan shipyard of Nanjing Tanker Corp. would be about to deliver the ship to its parent China Changjiang National Shipping (Group) Corp. after repair, the shipyard said in a statement.

Of the 12 shipyard workers who were admitted to a hospital, two sligtly injured have been discharged and the rest are still under emergency medical treatment, it said.

An investigation into the cause of the explosion is underway.