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A giant offshore crane salvages a large portion of the sunken South Korean naval ship Cheonan off Baengnyeong Island, South Korea, on Saturday. ((Jin Sung-chul/Yonhap/Associated Press))

Investigators say they have new information regarding last month's sinking of a South Korean naval ship that indicates the force of a passing torpedo may have caused the disaster.

Defence Minister Kim Tae-young told reporters Sunday he believes a bubble jet effect is likely to blame. The term is used for the destructive column of water created after a torpedo is launched.

Yoon Duk-Yong, co-chair of the joint investigation team, said officials arrived at this conclusion following initial inspections of the wreckage and added the results are not yet definitive. However, he said there was no possibility of an internal explosion or a collision with the reef.

Kim said it would take up to a month before the joint investigation team comes out with its final report.

The remarks bolster suspicions that North Korea's involvement is possible with a torpedo or a mine attack, although Seoul has not directly blamed Pyongyang for the blast and the North has denied any involvement.

The site of the sinking is near an area where the rival Koreas have 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.

The Cheonan was on a routine patrol on March 26 when an 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.

South Korea's coast guard was able to rescue 58 crew members from waters near the blast site, close to the disputed maritime border with North Korea.

On Saturday, a South Korean crew used a giant floating crane to lift the final half of the 1,200-tonne warship onto a barge off Baengnyeong Island.

The stern of the vessel was salvaged last week.