Defense Minister Kim Tae-young said military authorities have not ruled out North Korean involvement in the sinking of the Cheonan, which split apart within minutes of an explosion in the rear hull late Friday night, according to the ship's captain.
Fifty-eight crew members were rescued from the Yellow Sea waters near Baengnyeong Island west of Seoul, but 46 others are missing, most likely inside a rear segment of the ship, military officials said. Divers rapping on the stern with hammers got no response Monday, military officials said.
South Korean officials have been careful to say the exact cause of the explosion remains unknown, and that the rescue mission remains their priority.
However, Kim told lawmakers Monday that North Korean involvement was one possibility.
"North Korea may have intentionally floated underwater mines to inflict damage on us," he said.
The two Koreas remain in a state of war because their three-year conflict ended in a truce, not a peace treaty, in 1953. North Korea disputes the maritime border drawn by the United Nations in 1953, and the western waters not far from where the Cheonan went down have been the site of three bloody skirmishes between North and South.
A mine placed by North Korea during the Korean War may also have struck the ship, he said. Many of the 3,000 Soviet-made mines North Korea planted during the war were removed, but not all. Kim noted that a North Korean mine was discovered as recently as 1984.
There are no South Korean mines off the west coast, he added. Kim also ruled out a torpedo attack, citing rescued sailors who were manning the radars.
Officials have also said an internal malfunction may be to blame. The 1,200-ton Cheonan is designed to carry weapons, and was involved in a previous skirmish with North Korea.
U.S. and South Korea military officials said there was no outward indication that North Korea was involved in the sinking of the Cheonan.
However, "neither the government nor the defense ministry has ever said that there was no possibility of North Korea's involvement," Kim said.
The North Korean military was keeping a close watch on the search operation, the Joint Chiefs of Staffs said in a defense committee report cited by the Yonhap news agency.
But Pyongyang's state media have made no mention of the ship. The North Korean military's first comments since the ship went down warned the U.S. and South Korea on Monday against engaging in "psychological warfare" by letting journalists into the Demilitarized Zone.
President Lee Myung-bak said rescuers "should not give up hope" of finding the crewmen, according to a statement from the presidential Blue House after Lee met with a security ministers Monday.
"We'll continue our search operation until the last minute without giving up hope of rescuing even a single survivor," a Joint Chiefs officer said Monday on condition of anonymity in line with department policy.
But the prospect of pulling out anyone alive seemed dim Monday. Any navy crewmen who initially survived and managed to seal themselves inside watertight cabins would likely have run out of air by Monday night since the supply of oxygen in the cabins was estimated to last up to 69 hours.
Rough waves over the weekend prevented military divers from gaining access to the wreckage. On Monday afternoon, divers finally reached the ship's rear segment where most of the missing were believed to be trapped.
Divers knocked on the ship with hammers but there was no response, Rear Adm. Lee Ki-sik of the Joint Chiefs of Staff told reporters.
The U.S. Navy sent four ships and a team of divers to join the search, said Lt. Anthony Falvo, a spokesman for the U.S. 7th Fleet, based just south of Tokyo.
At a naval base south of Seoul, anguished relatives waited for news from the search mission, some pounding their chests with grief.
"My baby, My baby!" one woman murmured while being carried on a gurney to an ambulance after briefly losing consciousness.