Four sailors are missing after a navy vessel capsized off the southeast coast.

The Office of the Joint Chiefs of Staff said Wednesday that the navy ship was returning from a routine training mission when it sank 35 kilometers off Ulsan late Tuesday night. There is no word on what caused the ship to go down.

Military authorities say one of the five crew members was rescued. Navy rescue vessels aided by Air Force aircraft are searching the area for the missing sailors, but their fate is still not known.

The navy vessel was training in the waters to detect possible incursions by North Korean submarines and vessels.