Many of 46 missing crew members were presumed to be trapped in the bottom of the 1,200-ton Cheonan, South Korean news agency Yonhap reported.
Lee Ki-shik of South Korea's Joint Chiefs of Staff said crews were battling the clock because waterproof cabins in the sunken ship held enough air for only 69 hours after it went under.
The Cheonan broke up after an explosion in its hull while it was patrolling near the Yellow Sea border with North Korea Friday. Fifty-eight of the ship's 104 crew members, including the captain, were rescued.
South Korean President Lee Myung-bak ordered the military to use "all the manpower and equipment available to conduct a search operation as fast as possible" and not give up on the possibility of finding survivors, presidential spokeswoman Kim Eun-hye said.
South Korean officials say the cause of the explosion could have been a collision with a mine that might have either drifted from North Korean waters or been at that location since the Korean War in the 1950s, Yonhap said.
South Korean and U.S. officials both said they have detected "no unusual movements by North Korean forces."
South Korean naval officials said they plan to salvage the ship, which was carrying missiles and torpedoes, to determine what caused the explosion.