SEOUL: South Korea's president ordered the military on alert Tuesday for any moves by rival North Korea after the defense minister said last week's explosion and sinking of a South Korean ship may have been caused by a North Korean mine.

The blast ripped the 1,200-ton ship apart last Friday night during a routine patrol mission near Baengnyeong Island, along the tense maritime border west of the Korean peninsula. Fifty-eight crew members, including the captain, were plucked to safety; 46 remain missing with dim prospects for finding any further survivors.

The Joint Chief of Staff said the exact cause was unclear, and U.S. and South Korean officials said there was no outward indication of North Korean involvement.

However, Defense Minister Kim Tae-young told lawmakers Monday that a floating mine dispatched from North Korea was one of several scenarios for the disaster. "Neither the government nor the defense ministry has ever said there was no possibility of North Korea's involvement," Kim 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 sea border drawn by the United Nations in 1953, and the western waters near the spot where the Cheonan went down have been the site of three bloody skirmishes between North and South.

"Since the sinking took place at the front line, the military should thoroughly prepare for any move by North Korea," President Lee Myung-bak told his Cabinet, according to his spokesman, Park Sun-kyoo. "I want the military to maintain its readiness."

The Joint Chiefs of Staff said the cause of the blast may remain unclear until the ship is salvaged after the rescue operation is over.

Any navy crewmen who managed to seal themselves inside their watertight cabins would have run out of air by Monday night since the supply of oxygen in the cabins was estimated to last up to 69 hours, military officials said.

Hampered by rough waves, divers finally reached the ship's rear segment, where most of the missing were believed trapped, and pumped oxygen into cracks in the stern on Monday, according to the Joint Chiefs of Staff. They hammered on the hull but got no response.

Military divers were gearing up to break into the ship Tuesday, Rear Adm. Lee Ki-sik of the Joint Chiefs of Staff told reporters.

"The fate of the 46 young soldiers defending our territory in the border area is still unknown," President Lee said. "I cannot describe how regrettable this is. Let's not give up hope until the last moment"

The disaster is one of South Korea's worst. Kim said Monday that the ship may have struck a mine left over from the war or deliberately dispatched from the North.

Many of the 3,000 Soviet-made naval mines North Korea planted in the waters off both coasts during the war were removed, but not all. Kim noted a North Korean mine was discovered as recently as 1984.

"North Korea may have intentionally floated underwater mines to inflict damage on us," Kim told lawmakers.

He insisted there were no South Korean mines off the west coast, and ruled out a torpedo attack from North Korea, which would have been spotted by radar.

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.

Pyongyang's state media have made no mention of the ship.

Earlier Friday, North Korea's military warned of "unpredictable strikes" if the U.S. and South Korea attempted to topple the regime. On Monday, a military spokesman accused the countries of engaging in "psychological warfare" by letting journalists into the Demilitarized Zone.

Anguished relatives waited for news at a naval base in Pyeongtaek, south of Seoul, some pounding their chests with grief, others shouting their displeasure at the slow pace of the rescue.

"My baby, my baby," one mother murmured, briefly losing consciousness.

"The navy is rotten to the core!" a man yelled, lunging at soldiers standing arm in arm to block angry family members from barging into the command center.