U.S. South Korea
A U.S. fighter takes off from the flight deck of an aircraft carrier during joint military exercises between the U.S. and South Korea, July 26, 2010. Photo by AP
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U.S. and South Korean warships and helicopters practiced anti-submarine maneuvers off the Korean peninsula Monday that officers said they hope would serve as a warning to Pyongyang that aggression in the region would not be tolerated.

An international investigation has blamed the North for the sinking of a South Korean warship, killing 46 sailors in what officials called the worst military attack on the South since the 1950-53 Korean War.

The four-day "Invincible Spirit" exercises involving 20 ships and submarines, 200 aircraft and 8, 000 troops come were conducted amid flourishes of rhetoric typical of the North Korean regime, which vowed to respond with "a sacred war and a powerful nuclear deterrence."

They will face a costly consequence if they stick to the criminal activities ravaging peace and security on the Korean peninsula, North Korea's main Rodong Sinmun newspaper said in commentary carried Monday by the official Korean Central News Agency. North Korea says the investigation results were fabricated and has accused the United States of attempting to punish it.

Pyongyang's latest rhetoric was seen by most as bluster: South Korea's Defense Ministry said it had not observed any significant moves by the North Korean military since the maneuvers began Sunday.

But the threats carry extra weight following the sinking of the Cheonan that dramatically intensified tension on the Korean peninsula. The ship sank near the tense western sea border, a scene of three bloody inter-Korean maritime battles in recent years.

U.S. officials say that the maneuvers, held well away from North Korea's border, are not intended to provoke a response, but add that they do want to send Pyongyang a message that further aggression in the region will not be tolerated and that the alliance between the U.S. and South Korea remains strong.

The peninsula technically remains at state of war because an armistice that ended the 1950-53 Korean War was never replaced with a peace treaty. Tuesday marks the 57th anniversary of the signing of the cease-fire.

Cmdr. Ray Hesser, head of an anti-submarine helicopter squadron on the George Washington, said North Korean submarines are largely restricted to shallow, coastal waters.

"We're not expecting to see them out here," he said. "I would not think they would be willing or wanting to come all the way out here."