South Korea's defence minister confirmed Monday that traces of high explosive were found on the wreckage of a warship sunk by a mystery blast, indicating it was probably hit by a torpedo.
The 1,200-tonne corvette was split in two near the tense Yellow Sea border with North Korea on March 26. Suspicion has fallen on the North.
Defence Minister Kim Tae-Young, confirming earlier media reports that had been denied by his ministry, said investigators had found traces of RDX explosive, which is widely used in torpedoes.
The discovery means a torpedo was the likeliest cause of the disaster, Kim said without specifying who may have launched it. But "it is too early yet" to draw a clear conclusion, he added.
His ministry said the traces were found in the ship's funnel and in sand collected from the seabed.
Kim also said investigators had found metal fragments that did not appear to come from the ship.
Results of a multinational investigation into the sinking, which killed 46 sailors, are due next week, local media reports say. The South is pondering ways to respond if the North's involvement is proved.
A defence ministry official told AFP the South could resume its anti-Pyongyang loudspeaker broadcasts along the border, for the first time in almost six years.
The two Koreas in June 2004 suspended the broadcasts as part of peace efforts under Seoul's then-liberal government.
Relations deteriorated when conservative President Lee Myung-Bak took office in 2008 and adopted a tougher line on cross-border relations.
Lee hinted last Tuesday North Korea was involved in the sinking and promised a "resolute" response when the cause is established. The North has denied responsibility.
The South has not publicly ruled out a military response but said it would probably take the issue to the United Nations Security Council, which is empowered to impose punitive sanctions.
Local media reports said Seoul may also cut down on inter-Korean trade, especially items that could finance the North's military, and might block the North's freighters from using the Jeju Strait off South Korea's south coast.
A senior US official said key nations were united in watching the North as the Cheonan investigation unfolded, and reiterated that any resumption of six-nation talks on ending Pyongyang's nuclear programme would have to wait for the outcome of the probe into the sinking.
Deputy Secretary of State James Steinberg said he hoped China, North Korea's main ally, had pressed leader Kim Jong-Il on the Cheonan when he visited Beijing this month.
"We are determined to pursue this thoroughly and to follow the facts where they may point and this in turn will have an impact on how we proceed" in dealing with North Korea's nuclear programme, Steinberg said.
Investigators have said an underwater external explosion sank the ship. But Scott Snyder, a senior analyst with the Asia Foundation, said firm proof that it came from a North Korean torpedo was unlikely to emerge.
"The biggest challenge in mounting an effective response… is that as long as the case is circumstantial, there will be a surfeit of conspiracy theories, but no solid basis upon which to take retaliatory action," he wrote in a weblog for the US Council on Foreign Relations.
Seoul's defence ministry, in a briefing paper, debunked what it called a "false controversy" especially in cyberspace about the cause of the tragedy.
It said the Cheonan was on regular patrol at the time and claims of a grounding and a subsequent collision with a US warship "are completely baseless".
A joint US-South Korean naval exercise at the time was more than 160 kilometres (100 miles) away from the scene of the sinking, the ministry said.