The United States told North Korea Thursday it would be punished for sinking a South Korean warship, but kept troop readiness normal in a sign it did not expect an immediate military flare-up.

The torpedo attack, which cost 46 lives and prompted international calls for new sanctions against the nuclear-armed hermit state, was an "unprovoked and unwarranted" act of aggression, State Department spokesman Philip Crowley said.

"This was a serious provocation. There will be definitely be consequences because of what North Korea has done," Crowley told reporters, without specifying what those might be.

"This is abominable," he added. "It is not the way that, you know, civilized nations act toward one another."

North Korea has dismissed as "sheer fabrication" a report by international investigators into the March 26 incident that pins the blame on Pyongyang, but South Korea and major world powers agree there is no doubt about it.

The White House called the attack "a challenge to international peace and security and a violation of the armistice agreement" which ended the 1950-53 war.

Defense Secretary Robert Gates said the Pentagon was in "close consultation" with South Korea in preparing a response but that it was up to Seoul to decide how best to proceed.

His top officer Admiral Mike Mullen said the almost 30,000 American troops in South Korea were on a "normal" state of readiness and were operating "routinely" for the time being.

"The forces are clearly aware of what's gone on, but we haven't changed any readiness levels as a result of this, up to this point," Mullen said.

The State Department also said it should be up to Seoul to indicate what action it believes should be taken to punish Pyongyang, but did not rule out unilateral US actions as well.

"We will be guided by what South Korea, within its own government and pursuing its own interests, decides," said Crowley. "We are an ally and friend of South Korea, and we will support them whatever their choice is.

Crowley did not rule out putting North Korea back on a State Department list of countries deemed to support terrorism.

The previous administration of president George W. Bush removed North Korea from the list in a bid to revive nuclear disarmament talks with Pyongyang.

Blacklisting North Korea would pave the way for the US Congress to impose fresh sanctions on North Korea.

The report issued in Seoul on Thursday by a multinational team investigated the sinking of the 1,200-tonne corvette near the disputed inter-Korean border.

North Korea, which has repeatedly denied responsibility for sinking the warship, is warning of "full-scale war" if new sanctions are imposed, according to Seoul's Yonhap news agency.

The United States stations some 28,500 troops to bolster South Korea's 655,000-strong armed forces against the North's military of 1.2 million troops.

It also guarantees a "nuclear umbrella" over its long-time ally in case of a nuclear attack.

earlier related report
N.Korean torpedo sank S.Korean warship: investigators
Seoul (AFP) May 21, 2010 - South Korea accused North Korea Thursday of torpedoing a warship near their disputed border, sending regional tensions rising as the North responded with threats of war.

The South's President Lee Myung-Bak promised "resolute countermeasures" after a multinational investigation team said there was overwhelming evidence a North Korean submarine sank the ship on March 26 with the loss of 46 lives.

The United States, Britain, Australia, Japan and France strongly condemned Pyongyang. But China, whose backing would be crucial in any attempt to penalise the North, appealed for restraint and did not criticise its ally.

The North said the investigators' report was based on "sheer fabrication" and threatened "all-out war" in response to any attempt to punish it.

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

The White House called the attack "a challenge to international peace and security and a violation of the armistice agreement" which ended the 1950-53 war.

The State Department called the attack "unprovoked" and warned Pyongyang there would "definitely be consequences".

British Foreign Secretary William Hague condemned a "callous act" while Japan said the North's action was "unforgivable" and soured hopes of restarting six-party nuclear disarmament talks.

UN chief Ban Ki-moon described the facts in the report as deeply troubling while NATO said North Korea's action "constitutes a clear breach of international law."

France offered South Korea "full solidarity" and called on North Korea "to abandon the path of murderous violence."

The sinking caused outrage in South Korea, which declared five days of national mourning last month. Cross-border relations, which have been frosty for months, went into a deep chill.

But Seoul has apparently ruled out a military counter-strike for fear of igniting all-out war and is instead likely to ask the United Nations Security Council to slap new sanctions on its neighbour.

This would need agreement from China, a veto-wielding member.

"All parties should stay calm and exercise restraint," foreign ministry spokesman Ma Zhaoxu said in Beijing, adding that China would be making its own assessment of the investigation results.

Japan's Foreign Minister Katsuya Okada told reporters that "we would like China to take concerted action as much as possible."

President Lee told Australian Prime Minister Kevin Rudd in a phone conversation: "Resolute countermeasures will be taken against North Korea."

South Korea, through strong international cooperation, "should make North Korea admit its wrongdoing and return as a responsible member of the international community", added Lee, whose National Security Council will meet Friday to weigh its reaction.

The sinking of the Cheonan near the Yellow Sea frontier flashpoint was the worst inter-Korean incident since the North's downing of a South Korean airliner in 1987 with the loss of 115 lives.

But Kim Yong-Hyun, of Seoul's Dongguk University, said the South has "not that many effective options" for a response if China fails to support UN action.

The 1,200-tonne corvette was split apart by a shockwave and bubble effect produced by a 250 kilogramme (550 pound) homing North Korean torpedo, the report said.

It said parts salvaged from the Yellow Sea "perfectly match" a type of torpedo that the North has offered for export and at a press conference investigators showed evidence that included torpedo parts with two propellers.

The report said the attack was likely carried out by a small submarine which infiltrated from international waters to avoid detection.

"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 report said.

The North's top organ, the National Defence Commission, 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," the North said.

It threatened in future to respond to any small border incident with a "merciless strong physical blow".