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.