SEOUL, South Korea (AP) — An explosion from a torpedo likely sank a South Korean warship that went down near the tense border with North Korea last month, the South's defense minister said Sunday amid growing speculation Pyongyang may be behind the blast.

Defense Minister Kim Tae-young said the most likely cause of the disaster was a torpedo exploding near the ship, with the force of the underwater blast ripping the vessel apart. 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 for the blast, Kim told reporters.

Kim, however, did not speculate on who may have fired it and said an investigation was ongoing and it's still too early to determine the cause.

Seoul has not directly blamed North Korea for the blast, and Pyongyang has denied its involvement, but suspicion remains given the country's history of provocation and attacks on the South. Local media have increasingly been speculating the disaster was due to a North Korean attack.

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 North Korean 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.

Kim told lawmakers shortly after the blast that a North Korean torpedo or mine was among the suspected culprits. He said earlier this month that a torpedo attack was more likely.

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.

"In conclusion, after the initial visual inspection of the severed surface and the inside and outside of the hull, we assume the case is underwater explosion," chief investigator Yoon Duk-yong told reporters. "And looking at the form of the deformation, it is highly likely that a non-contact explosion was the case rather than a contact explosion."

But he said it's still 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 to the sailors.

"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.

In Pyongyang, the North marked the 78th anniversary of the foundation of the country's military Sunday with a vow to "mercilessly" punish any hostile moves by "the imperialist enemies," a term it uses when referring to the U.S.

Pyongyang routinely accuses the U.S. of plotting to invade the North, despite the repeated denials by Washington.

"If the imperialist enemies intrude into" the North's territory, "its army will beat them back at a stroke by mercilessly showering bombs and shells on them," the North's main Rodong Sinmun newspaper said in an editorial carried by the official Korean Central News Agency. It didn't mention the ship sinking.