Hong Kong and Vietnamese rescue workers are searching for survivors after a South Korean cargo ship sank in the South China Sea, with five South Korean and four Myanmar crew still unaccounted for, South Korea\'s News Agency (Yonhap) reported Tuesday.The 15,000-ton bulk carrier was carrying 21 crew when it went down on Monday afternoon while sailing from Malaysia to mainland China. Twelve crew members, including four South Koreans, were rescued by other commercial ships at the scene, foreign ministry officials said. \"We are closely cooperating with Hong Kong and Vietnamese maritime police to search for the missing crew,\" a ministry official said on the condition of anonymity. Hong Kong police are mobilizing their helicopters to search for survivors near the scene, the official said. It was not immediately clear why the ship, named \"Bright Ruby,\" sank. Seoul officials said the ship went down some 350 nautical miles south of Hong Kong waters.The ship was hijacked by Somali pirates off the Gulf of Aden in September 2008, ministry officials said. The ship and its eight South Korean crewmen were released after 37 days in captivity.