– The Associated Press

SEOUL, South Korea – South Korea’s president ordered the military on alert today 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.

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.

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 today, Rear Adm. Lee Ki-sik told reporters.

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.

Officials have also said an internal malfunction may be to blame.