Experts investigating the blast that sank a South Korean warship are checking salvaged metal fragments against a stray North Korean torpedo Seoul retrieved years ago, a report said Thursday.

South Korea has mounted a multinational probe into what caused the explosion that split the 1,200-tonne corvette the Cheonan in two near the disputed border with the North on March 26.

"Comparisons are under way to check if metal pieces recovered from the Cheonan are made of material similar to that of a North Korean torpedo," an unidentified military official told Yonhap news agency.

The official was quoted as saying the South's military obtained a North Korean torpedo off the west coast in 2003. The official did not elaborate and the defence ministry declined comment.

Defence Minister Kim Tae-Young earlier confirmed that traces of high explosive were found on the warship wreckage, indicating it was probably hit by a torpedo.

The minister also said metal fragments that did not appear to come from the ship had been found.

The investigation into the sinking, which killed 46 South Korean sailors, is due to report next week. The South is expected to take punitive action if the North, which has denied responsibility, is proved to have sunk the corvette.

President Lee Myung-Bak, who took office in 2008 and adopted a tougher line on cross-border relations, hinted last week North Korea was involved and promised a "resolute" response if this proved to be the case.

Unification Minister Hyun In-Taek dropped a similar hint Thursday.

"The sinking of the Cheonan has shown the world the cruel reality of division" on the Korean peninsula, he said in a speech to a forum.

Officials have said Seoul will try to report Pyongyang to the United Nations Security Council for possible further sanctions.

Local media reports have said it may reduce 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.

The unification ministry, which handles cross-border relations, declined to comment on a report by Chosun Ilbo newspaper that the ministry has worked out a package of sanctions.

The paper said these could include a ban on imports of sand and fisheries products from the North.

Trade in sand for construction was worth 70 million dollars in 2008 to the North, and trade in fisheries products was worth 173 billion won (151 million dollars) in 2009, Chosun said.

Yonhap said the government is also urging some South Korean companies to halt new investment in a jointly-run industrial estate at Kaesong north of the border, and not to sign new business deals.

The South is also considering resuming loudspeaker propaganda broadcasts along the border, for the first time in almost six years.