A torpedo fired by a North Korean submarine sank a South Korean warship near the disputed maritime border with the loss of 46 lives, investigators said Thursday.

"The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine," the multinational team said in its report on the March 26 sinking.

"There is no other plausible explanation."

President Lee Myung-Bak promised "resolute countermeasures" to the attack, which sparked national mourning in the South, but gave no immediate details.

The White House called the report "objective and scientific".

"The United States strongly condemns the act of aggression that led to their (sailors') deaths," said spokesman Robert Gibbs.

The communist North's top decision-making body repeated denials of involvement and threatened "all-out war" in response to any punitive measures such as sanctions.

The investigators' report said torpedo parts salvaged from the Yellow Sea "perfectly match" a type of torpedo that North Korea has offered for export.

It said a marking in Korea's Hangeul script was found on one recovered section, and matched markings on a stray North Korean torpedo recovered by the South seven years ago.

The North dismissed claims of its involvement in the sinking -- the worst apparent cross-border provocation since the downing of a South Korean airliner in 1987 with the loss of 115 lives -- as a "fabrication".

But the investigators, including experts from the United States, Australia, Britain and Sweden, laid out apparently damning evidence.

The ship was split apart by a shockwave and bubble effect produced by the underwater explosion of a 250 kilogramme (550 pound) homing North Korean torpedo, the report said.

It said torpedo parts recovered by a dredging ship on May 15th -- including the propellers, propulsion motor and a steering section -- "perfectly match the schematics of the CHT-02D torpedo included in introductory brochures provided to foreign countries by North Korea for export purposes".

The report said the attack was likely carried out by a small submarine.

"We confirmed that a few small submarines and a mother ship supporting them left a North Korean naval base in the West (Yellow) Sea 2-3 days prior to the attack and returned to port 2-3 days after the attack."

The sinking caused outrage in South Korea, which declared five days of national mourning. But Seoul is believed to have ruled out a military counter-strike for fear of igniting all-out war.

It is likely to ask the United Nations Security Council to slap new sanctions on the North, in addition to those imposed to curb its missile and nuclear programmes.

China, a veto-wielding council member and the North's ally, is unlikely to support this unless it accepts the South's evidence.

The South could also restrict trade with its neighbour and ban the North's merchant ships from using the Jeju Strait off the peninsula's south coast.

"Resolute countermeasures will be taken against North Korea," Lee told Australian Prime Minister Kevin Rudd in a phone conversation, according to a statement from Lee's office.

South Korea, through strong international cooperation, "should make North Korea admit its wrongdoing and return as a responsible member of the international community", Lee added.

Lee said the North in the past had committed "military provocations or terrorist acts against South Korea and denied involvement".

This time there was evidence "that no one or no country in the world can deny", said Lee, whose National Security Council will meet Friday.

The North's National Defence Commission, in a statement on the official news agency, said it would send its own investigators to the South to check the purported evidence.

"Our army and people will promptly react to any 'punishment' and 'retaliation' and to any 'sanctions' infringing upon our state interests with various forms of tough measures including an all-out war, it said.

North and South Korea have remained technically at war since their 1950-53 conflict ended only in an armistice.