BAENGNYEONG ISLAND, South Korea -- 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 that sank 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 near Baengnyeong Island near the tense maritime border west of the divided Korean peninsula. Fifty-eight crew members, including the captain, were plucked to safety; 46 are missing, with dim prospects for their survival.

A 53-year-old diver who lost consciousness during the underwater mission to locate the missing crewmen died Tuesday, the Joint Chiefs of Staff said. A second diver was being treated for injuries, officials said.

As the search continued, divers prepared to break into the ship Tuesday, Rear Adm. Lee Ki-sik of the Joint Chiefs of Staff told reporters. He said there were no signs of life inside.

President Lee Myung-bak flew to wreckage site to review search operations, meet with marines and console family members, the presidential Blue House said.

Baengnyeong is just eight miles (13 kilometers) from and within sight of a North Korean military base where surface-to-ship guided missiles and artillery are heavily deployed, presidential spokesman Park Sun-kyoo said.

Lee told officers South Korea must maintain its military readiness until North Korea gives up its nuclear weapons program.

"When we are strong, we can defend ourselves. If we are weak, we'll face more danger," Lee said. "South Korea's military should be strong."

Earlier Tuesday, Lee ordered his military to stay alert for any moves by rival North Korea.

"Since the sinking took place at the front line, the military should thoroughly prepare for any move by North Korea," Lee told his Cabinet, according to his spokesman.

Military officials say the exact cause of the explosion remains unclear, and U.S. and South Korean officials said there was no evidence 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.

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