South Korea's military on alert after ship sinks

South Korea's president ordered the military on alert Tuesday for any moves by rival North Korea after the defence minister said last week's explosion that sank a South Korean ship may have been caused by a North Korean mine.

The blast ripped theCheonan apart last Friday night during a routine patrol near Baengnyeong Island, not far from the tense maritime border west of the divided Korean peninsula.

Fifty-eight crew members, including the captain, were rescued 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 crew members died Tuesday, the Joint Chiefs of Staff said. A second diver was being treated for injuries.

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 visits wreckage site

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

Baengnyeong is just 13 kilometres from 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."

"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.

Exact cause unknown

Military officials said 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, Defence 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 the two Koreas.