South Korea is to formally blame North Korea on Thursday for launching a torpedo at one of its warships in March, causing an explosion that killed 46 sailors and heightened tensions in one of the world's most perilous regions, U.S. and East Asian officials said.

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South Korea is to formally blame North Korea on Thursday for launching a torpedo at one of its warships in March, causing an explosion that killed 46 sailors and heightened tensions in one of the world’s most perilous regions, U.S. and East Asian officials said.

South Korea reached its conclusion that North Korea was responsible for the attack after investigators from Australia, Britain, Sweden and the United States pieced together portions of the ship at the port of Pyongtaek, 40 miles southwest of Seoul. The Cheonan sank on March 26 after an explosion that rocked the vessel as it sailed in the Yellow Sea off South Korea’s west coast.

Chosun Ilbo and other South Korean newspapers reported Tuesday that investigators found fragments of what appeared to be a Soviet- or Chinese-made torpedo near the wreck.

The U.S. and East Asian officials, who spoke on condition of anonymity because South Korea has yet to disclose the findings of the investigation, said that subsequent analysis determined the torpedo was identical to a North Korean torpedo that had previously been obtained by South Korea.

South Korea’s conclusion underscores the continuing threat posed by North Korea and the intractable nature of the dispute between the two Koreas. They remain technically at war, because their three-year conflict ended in a truce, not a peace treaty, in 1953. South Korea has said it will take stern action against anyone responsible for the sinking.

South Korean President Lee Myung-bak must respond forcefully to the attack, analysts said, but not in a way that would risk further violence from North Korea, whose artillery could — within minutes — devastate greater Seoul, which has a population of 20.5 million.

South Korea will request that the U.N. Security Council take up the issue and is looking to tighten sanctions on North Korea, the officials said. The United States has indicated it would support such an action, U.S. officials said.

Another consequence of the report, experts predicted, is that Lee will request that the United States delay for several years a plan to pass operational control of all forces in South Korea from the United States to the South Korean military. Approximately 28,500 U.S. forces are stationed in South Korea.

South Korea’s conclusion that North Korea was responsible for the sinking of the Cheonan also means it is unlikely that talks will resume anytime soon over North Korea’s nuclear-weapons program. North Korea has twice tested what is believed to be a nuclear weapon.

The sinking — and the reluctance of the South to respond with an in-kind attack — is the latest example of the raw military intimidation that North Korea has practiced for decades. With 1.19 million troops on active duty, the Korean People’s Army has positioned about 70 percent of its fighting forces and firepower within 60 miles of the border with the South.

Additional information from The Associated Press was included in this report.