The 1,200-ton Cheonan went down in the Yellow Sea on March 26 after an explosion tore through the frigate. Fifty-eight sailors were rescued, but 46 were killed. Seoul has not directly blamed North Korea for the sinking, and Pyongyang has denied involvement. However, suspicion has focused on the North, given its history of provocations and attacks.
South Korean Defense Minister Kim Tae-young said earlier that an initial investigation indicated that a torpedo was likely to blame for the disaster. On Monday, he confirmed media reports that traces of a high explosive were found on the ship's wreckage.
"It's true that RDX, a chemical substance used in making torpedoes, has been detected," he told reporters. "The possibility of a torpedo (attack) has increased, but it's too early to say anything."
The explosive material was detected on the ship's smokestack and in samples of sand from the site, said joint investigation team spokesman Rear Adm. Moon Byung-ok.
"Mines use RDX as well as torpedoes so we need to investigate further to determine which was responsible," he said.
RDX, or research department explosive, is a powerful high explosive often mixed with other explosives such as TNT or TORPEX, and has been widely used since World War II by all countries for military and industrial purposes, Kim said.
The multinational investigation team is analyzing aluminum pieces salvaged from the wreckage to determine whether they are a part of the ship or a torpedo, Kim said.
"It is not easy to tie the debris to weapons like torpedoes because information on other countries' weapons systems is not readily available," he said.
Reports say investigation results are expected within weeks, but officials have said there is no deadline.
The two Koreas remain locked in a state of war because their three-year conflict ended in a truce, not a peace treaty, in 1953.
North Korean artillery sank a South Korean ship in 1967, killing 39 sailors, and their navies have fought three more bloody skirmishes off the west coast since 1999.
Pyongyang also is suspected in a 1983 bombing in Burma that narrowly missed killing then-South Korean President Chun Doo-hwan but left 21 others dead, as well as for the 1987 bombing of a South Korean airliner that killed 115 people on board.