President Lee Myung-bak vowed "stern action" for the provocation following the release of long-awaited results from a multinational investigation into the incident. North Korea, reacting swiftly, called the results a fabrication and warned that any retaliation would trigger war.

Investigators said evidence overwhelmingly proves North Korea fired a homing torpedo that caused a massive underwater blast that tore the Cheonan into two on March 26. Fifty-eight sailors were rescued from the frigid Yellow Sea waters near the Koreas' maritime border, but 46 perished.

"(We) will take resolute countermeasures against North Korea and make it admit its wrongdoings through strong international cooperation," Lee told Australian Prime Minister Kevin Rudd in a phone conversation, the presidential office said.

The White House called the sinking an unacceptable "act of aggression" that violates international law and the truce signed in 1953.

U.N. Secretary-General Ban Ki-moon, a former South Korean foreign minister, called the investigation results "deeply troubling," his spokesman said in a statement.

China, North Korea's traditional ally, called the sinking of the naval ship "unfortunate" but stopped short of backing Seoul. Vice Foreign Minister Cui Tiankai refused to comment further Thursday other than reiterating long-standing Chinese comments on the need to maintain peace on the Korean peninsula.

South Korean and U.S. officials have said they are considering a variety of options, ranging from U.N. Security Council action to additional U.S. penalties.

North Korea already is chafing from international sanctions tightened last year in the wake of widely condemned nuclear and missile tests.

Pyongyang, meanwhile, continued its steadfast denials of involvement in the sinking and said it would send its own investigators to conduct a probe, while warning that any punishment against the North would spark war.

"The all-out war to be undertaken by us will be a sacred war involving the whole nation, all the people and the whole state," a spokesman for North Korea's powerful National Defense Commission said, according to a report carried by the state-run Korean Central News Agency.

North Korea's Kim Jong Il serves as chairman of the National Defense Commission, a position that makes him leader of the communist nation of 24 million.

The North also warned the South against any provocative acts near the Koreas' borders in the aftermath of the sinking, saying it would react with an "unlimited retaliatory blow, merciless strong physical blow."

Pyongyang, which accused Lee's government of exploiting the disaster for political gain, also urged the U.S. and Japan to "act with discretion."

"The world will clearly see what dear price the group of traitors will have to pay for the clumsy 'conspiratorial farce' and 'charade' concocted to stifle compatriots," KCNA said.

The two Koreas remain locked in a state of war and divided by the world's most heavily armed border because the conflict ended with the signing of a truce, not a peace treaty.

North Korea has waged a slew of attacks against South Korea since the war, including the 1987 downing of a South Korean passenger plane that killed all 115 people on board.

Pyongyang routinely denies the past provocations.

North Korea also disputes the maritime border drawn unilaterally by U.N. forces at the close of the Korean War, and the waters have been the site of several deadly naval clashes since 1999.

Fragments recovered from the waters where the Cheonan went down indicate that a North Korean submarine fired a torpedo, investigators said Thursday.

Pieces recovered at the sinking site "perfectly match" the schematics of the torpedo included in introductory brochures provided to foreign countries by North Korea for export purposes, chief investigator Yoon Duk-young said.

A serial number on a torpedo fragment also was consistent with markings from a North Korean torpedo that South Korea obtained years earlier, Yoon said.

"The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine," he said. "There is no other plausible explanation."

Investigators also confirmed that several small North Korean submarines and a mother ship supporting them left a North Korean naval base two to three days ago before the attack, and returned to port two to three days after the attack.

Other nations' submarines were either in or near their respective home bases at the time of the incident, Yoon said.

The joint civilian-military investigation team included experts from South Korea, the U.S., Britain, Australia and Sweden.