South Korea’s president ordered the military to be on alert on Tuesday for any moves by North Korea after the defence minister said a mine from the rival country may have caused the explosion that sank a South Korean naval ship.

The blast ripped the 1,200—ton ship apart on 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 on Tuesday, the Joint Chiefs of Staff said. A second diver was being treated for injuries, officials said.

President Lee Myung—bak expressed his condolences, calling the death “unfortunate and regrettable,” according to his office.

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, Defence Minister Kim Tae-young told lawmakers on 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,” Mr. Kim said.

A North Korean defector who used to work for the isolated regime’s spy agency also suggested that it could have been the work of a suicide attacker.

As the search continued, divers attempted to break into the ship but made little headway due to strong currents and poor underwater visibility, Rear Adm. Lee Ki—sik of the Joint Chiefs of Staff told reporters.

Divers, aided by a team from four U.S. Navy ships in the region, rapped on the stern with hammers on Monday afternoon but got no response, they said. Aware that the crew would have only enough oxygen in their watertight cabins to last until Monday evening, divers pumped oxygen into the ship through cracks in the stern, Rear Adm. Lee said.

President Lee flew to the wreckage site on Tuesday to review search operations, meet with marines and console family members, the presidential Blue House said in a statement.

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 deployed, the statement said.

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

“When we are strong, we can defend ourselves. If we are weak, we’ll face more danger,” Mr. Lee said. “South Korea’s military should be strong.”

Earlier on Tuesday, Mr. 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,” Mr. Lee told his Cabinet, according to his office.

South Korea’s navy chief of staff Kim Sung—chan told Mr. Lee on Tuesday that there was no evidence the explosion was from the ship’s ammunition dump. Mr. Kim also said the military does not rule out a possibility that the explosion may have been caused by a torpedo attack, according to South Korean media pool reports.

The Cheonan is designed to carry weapons, and was involved in a previous skirmish with North Korea.

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.

North Korean navy suicide squads known as “human torpedoes” could also be behind the explosion, wrote a North Korean defector living in Seoul in a post on his personal blog.

Chang Jin—seong, a poet who worked for the North’s spy agency before he fled the country in 2004, wrote that some North Korean navy combat units train specifically for suicide attacks.

“Marines are trained to drive the bombs toward the target,” Mr. Chang said.

The defence minister said the ship may also have struck a mine left over from the 1950s war. Many but not all the 3,000 Soviet—made naval mines North Korea planted during the war were removed, and a mine was discovered as recently as 1984, Mr. Kim said.

He insisted there were no South Korean mines off the west coast.

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