An explosion caused by a torpedo likely tore apart and sank a South Korean warship near the Democratic People's Republic of Korea (DPRK) border, Seoul's defense minister said Sunday, while declining to assign blame for the blast on Pyongyang.

Defense Minister Kim Tae-young said 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 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. 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 DPRK torpedo was one of the likely scenarios, but the government has been careful not to blame the North outright, and Pyongyang has denied its involvement.

The Cheonan was on a routine patrol on March 26 when the unexplained explosion split it in two 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.

The site of the sinking is near where the rival Koreas fought three times since 1999, most recently a November clash that left one DPRK soldier dead and three others wounded. The two Koreas are still technically at war because their 1950-53 Korean War ended in a truce, not a peace treaty.

Also Sunday, investigators said a preliminary investigation of the front part of the 1,200-ton ship retrieved the day before pointed to an external explosion.

Chief investigator Yoon Duk-yong told reporters that an inspection of the hull pointed to an underwater explosion. He appeared to support the bubble jet effect theory, saying, "It is highly likely that a non-contact explosion was the case rather than a contact explosion."

But he, too, said it was too early to determine what caused the explosion.

Earlier Sunday, Prime Minister Chung Un-chan said South Korea will take "stern" action against whoever was behind the explosion as the country started a five-day funeral for the 46 dead and missing sailors. Makeshift alters were set up in Seoul and other major cities to allow citizens to pay their respect.

"We will remember all of you in the name of the Republic of Korea to let you keep alive in our hearts," said Chung, clad in a black suit and tie. The 46 sailors will be promoted by one rank and awarded posthumous medals, he said.