Seoul , South Korea ( CNN ) -- An experienced diver and master sergeant in the South Korean navy died Tuesday while conducting rescue efforts for 46 seamen who were aboard a patrol ship when it sank , a military official said . The 53-year-old chief master sergeant lost consciousness while underwater in the Yellow Sea , the official said . He was taken aboard a U.S. Navy ship , the USS Salvor , but could not be resuscitated . The 1,200-ton patrol ship Cheonan was carrying 104 sailors when an apparent explosion caused it to capsize Friday night in the Yellow Sea near a disputed maritime border between North and South Korea . Fifty-eight sailors were rescued shortly after the ship capsized , but hopes are fading for the remaining 46 . No bodies or survivors have been found since the 58 . U.S. divers were on the scene Tuesday and were observing the situation , according to the official . The U.S. divers plan to enter the water on Wednesday , he said . Efforts were under way to pump air into the ship 's stern , and ropes were also being attached to the ship to provide divers with some guidance while underwater , the official said . Visibility is poor -- only about a meter ( 3 feet ) -- and the current is about 5 knots ( 5 mph ) . South Korean divers were struggling to gain access to stern of the ship , where most of the missing crew members are believed to be , South Korea 's Yonhap news agency reported Tuesday . Defense officials told the news agency access to the stern was hampered by the current . Monday evening was the end of a 69-hour window during which rescuers believed the sailors could survive . Divers have knocked on the hulls of different parts of the ship with hammers , but raised no response . South Koreans have held out hope the crewmen might be rescued , but by Tuesday hopes for a happy ending were fading . Meanwhile , officials were no closer Tuesday to determining the cause of the explosion that sunk the Cheonan . The country 's defense minister has said it could have been caused by an old mine . Kim Tae-young told a meeting of the parliamentary defense committee on Monday that one of the many North Korean sea mines placed during the 1950-53 Korean War could have triggered the blast . Yonhap has quoted military officials as saying that the explosion tore a hole in the rear of the ship , shutting off the engine . Citing accounts of rescued sailors who handled the ship 's radar , Kim said they reported no signs of a torpedo attack ahead of the explosion . The navy plans to salvage the vessel , which was carrying missiles and torpedoes , to determine what caused the incident , Yonhap reported . President Lee Myung-bak has called for a thorough investigation into the cause of the explosion . Baengyeong Island , the Seoul-administered island near the scene of the accident , is a flash point maritime border area between the Koreas . Given Baengyeong Island 's proximity to North Korea , North Korean involvement was feared , but South Korean officials have played down that scenario . North Korea 's official media has yet to mention the incident , according to Yonhap , but accused the United States and South Korea of conducting a maritime drill for the purpose of invading North Korea . `` Military tension is intensifying on the Korean Peninsula due to the brazen moves by the U.S. and its war-mongering puppets to provoke a war , '' North Korea 's official Korean Central News Agency said , as reported by Yonhap . Journalist Andrew Salmon contributed to this report for CNN .
NEW : U.S. divers plan to enter water on Friday , official says . Diver lost consciousness while underwater during rescue and could not be revived . Rescue teams searching for 58 North Korean sailors missing after Wednesday explosion . 46 seamen were rescued soon after blast ; no bodies or survivors found since then .