Seoul: South Korea's president yesterday bemoaned the "precious deaths" of dozens of sailors killed when an explosion destroyed their warship, as suspicion increasingly fell on a North Korean torpedo blast in the disaster.

Makeshift altars have been set up across South Korea for a five-day mourning period that opened on Sunday to honour victims aboard the 1,200-tonne Cheonan when it split into two in an unexplained explosion on March 26. President Lee Myung-bak visited one of the shrines at a plaza in Seoul yesterday to pay his respects.

"The Republic of Korea will never forget your precious deaths," Lee wrote at a condolence book at the mourning station, according to South Korean media pool reports.

Last week, a tearful Lee said in a nationally televised speech that South Korea will respond "resolutely and unwaveringly" against anyone responsible for the blast.

On Sunday, Defence Minister Kim Tae-young said that an underwater explosion appeared to have ripped apart the vessel, and a torpedo blast seemed the most likely cause. Investigators who examined salvaged wreckage separately announced on Sunday that a close-range, external explosion likely sank it.

"Basically, I think the bubble jet effect caused by a heavy torpedo is the most likely" cause, Kim told reporters on Sunday. The bubble jet effect refers to the rapidly expanding bubble an underwater blast creates and the subsequent destructive column of water unleashed.

Kim, however, did not speculate on who may have fired the weapon and said an investigation was ongoing and it's still too early to determine the cause. Soon after the disaster, Kim told lawmakers that a North Korean torpedo was one of the likely scenarios.

As investigations have pointed to an external explosion as the cause of the sinking, however, suspicion of the North has grown, given its history of provocation and attacks on the South.

The Cheonan was on a routine patrol before the explosion sank it in one of South Korea's worst naval disasters. Forty bodies have been recovered so far, but six crew members are still unaccounted for and are presumed dead.