ELEVEN Vietnamese seamen are feared dead after their ship sank following a collision with another vessel in the South China Sea off Hongkong yesterday. A search and rescue mission by emergency services from Hongkong and China began within minutes of the Marine Department receiving a radio message from the 4,733-tonne Panamanian-registered cargo ship the Hing Son, after it was involved in a collision with the 327-tonne Hai Long. The accident happened 13 nautical miles south of Fan Lau, southwest of Lantau, shortly before 4 am yesterday. Duty officer of the Marine Department's Maritime Rescue Co-ordination Centre, Mr Lam Kit, said the captain of the Hing Son reported that the smaller vessel had sunk. Some of the 24 mainland Chinese crewmen on board the larger ship, which was bound for Hongkong, immediately threw life buoys into the water and picked up five of the sailors. An Auxiliary Air Force Sikorsky helicopter winched a 36-year-old man from the sea and flew him to the British Military Hospital. He was later transferred to Queen Elizabeth Hospital suffering from hypothermia and was said to be in fair condition. The Hing Son, which had a half-metre hole on its bow, sailed into Hongkong late yesterday. A 30-year-old Vietnamese was taken to Queen Elizabeth Hospital and was said to be in satisfactory condition. The remaining four survivors were taken ashore by a Marine Police launch and the vessel's agent arranged accommodation for them. The missing men are said to be aged between 20 and 48 and an Auxiliary Air Force spokesman said they were all without lifejackets or any specialist clothing. The British Garrison scrambled two Wessex helicopters from the Royal Air Force base in Sek Kong and the Royal Navy patrol craft HMS Plover acted as a floating command centre at the scene during the search. Two Royal Navy medics were airlifted to the Plover and put on standby to treat any casualties. A fast patrol craft dispatched from the Plover made a sweep of the area and recovered debris believed to be from the sunken ship. One vessel from the Guangdong Rescue Centre also took part in the search for survivors as the collision took place in Chinese territorial waters. At least two merchant ships, in the area at the time of the collision, also helped in the operation, which was finally called off at 3.10 pm. An Auxiliary Air Force fixed-wing Super King Air plane, piloted by Flight Lieutenant Bob Patrick, spotted the survivor in the water shortly after 6 am and he was winched aboard one of the two Sikorsky S76s an hour later. The helicopter pilot involved in the rescue said visibility was poor for the five hours of the air search. Flight Lieutenant Jack Stokes said: ''Although the sea was fairly calm, patchy fog was our biggest problem. It was a bit tricky, but we were able to manage fairly well. After the man was picked up, our helicopters and the RAF made more than 20 sweeps of the area but once we passed the four or five-hour mark after the sinking, the chances of finding anybody alive were very slim.'' Mr Lam said that after obtaining permission to go into Chinese waters to search for survivors, ''we immediately alerted the HMS Plover, the RAF and the Auxiliary Air Force''. ''Hing Son and two other vessels in the vicinity also joined in the search and rescue operation,'' he said. ''I also understand that Guangdong Rescue Co-ordination Centre had also mobilised their vessel Sui Jiu 204 to the scene. ''We searched a wide area of between 70 and 100 square nautical miles surrounding the scene, and so far no further survivors have been found.'' He said the weather was generally good, despite a cloudy sky, and the sea was slight when the accident occurred. ''We don't know how the two vessels collided and which part of the ships had been damaged during the crash,'' he said. ''I don't think we will investigate the incident because the vessels are not Hongkong-registered, no Hongkong seamen are involved and lastly, it happened within Chinese waters.'' The pilot of the first RAF helicopter which arrived at the scene, Flight-Lieutenant Jim Sweatman, said he saw a lot of wreckage from the sunken vessel and an empty life jacket floating in the sea. ''There were also television sets, gas cylinders, fuel cans and pieces of wood floating in the water,'' he said. ''Wind was light and sea condition was slight with one to 11/2 metres of swell and a visibility of three miles.'' The RAF ended its two-hour search at 9.15 am, but Auxiliary Air Force planes remained at the scene until 10.35 am.