ABOARD THE USS CURTIS WILBUR (AP) -- On the 57th anniversary of the armistice that ended the Korean War, U.S. and South Korean ships intensified high-profile military exercises Tuesday that underscore rising tensions in a region yet to truly find peace.
The massive maneuvers, called "Invincible Spirit," are being conducted by an armada of South Korean and U.S. ships, including the USS George Washington supercarrier, in international waters off the South Korean coast.
The normally quiet patch of the East Sea/Sea of Japan has been a buzz of military activity, with helicopters dropping sonar buoys into the waters, squadrons of carrier-based F-18 fighters embarking on bombing runs and destroyers blasting their guns at unmanned aerial drones.
The exercises come just four months after a South Korean warship was sunk, allegedly by a torpedo from a North Korean submarine, in the worst attack on the South's military since the Korean War ended in a shaky truce on July 27, 1953.
North Korea strongly has denied attacking the ship in which 46 sailors died. It has threatened to retaliate over the four-day maneuvers that end on Wednesday, but senior officers aboard the U.S. ships said no North Korean military activity has been observed.
The Koreas technically remain at war because fighting ended in a truce, not a peace treaty. On Tuesday, the Koreas and the United States marked the anniversary of the signing of the armistice.
"Since our nation's founding, the United States has relied on our armed forces to ensure our safety and security at home, and to protect lives and liberties around the globe," President Obama said in a statement. "I call upon all Americans to observe this day with appropriate ceremonies and activities that honor our distinguished Korean War veterans."
Gen. Walter Sharp, chief of 28,500 troops in South Korea, said the region remains tense, but he stressed that the U.S.-South Korean alliance is strong.
"The Korean peninsula has not been entirely at peace for the past 57 years," Gen. Sharp said during a ceremony at the Korean border village of Panmunjom. He said the United States and South Korea are ready to "defeat any future provocation from North Korea.
"We are stronger than we have ever been," he said.
Meanwhile, soldiers in Pyongyang, North Korea's capital, marked the anniversary of the "Fatherland Liberation War" by laying bouquets at a memorial. Residents held rallies and danced, according to the official Korean Central News Agency.
North Korean army officer Hong Sung Chol told APTN in Pyongyang the naval maneuvers were an attempt to stifle his country.
"The U.S. imperialists, together with their South Korean puppets, are finally staging large-scale joint military exercises in the East Sea of Korea, despite the strong opposition of the Korean and world people," he said.
The regime also pressed its demand for a full peace treaty. North Korea wants to establish diplomatic relations with Washington to guarantee the United States won't invade and topple Kim Jong-il's dynastic communist government.
Pyongyang cites the presence of U.S. troops in South Korea as a key reason for building up its atomic weapons program. The United States repeatedly has said it has no intention of attacking the North.
But this week's exercises were intended to warn the North that Seoul and Washington will not tolerate acts of aggression.
The exercises involve about 20 ships, 200 aircraft and about 8,000 U.S. and South Korean sailors. At the center of the maneuvers is the USS George Washington, a floating fortress that can carry up to 70 aircraft and more than 5,000 sailors and aviators.
As fighters flew off the decks of the carrier and South Korea's defense minister toured the ship, about a half-dozen destroyers stepped up their part of the drills by dropping anti-submarine bombs and firing artillery at target drones.
"It's a show of force, a deterrent," said Capt. Paul Hogue, commanding officer of the USS Curtis Wilbur. He said the exercises have been a success.
"I think it's gotten their attention," he said.
Lt. Kim Seong-joon, a South Korean liaison officer aboard the Curtis Wilbur, said the sinking of the corvette Cheonan in March was a wake-up call for his nation. He said he lost two friends in the sinking.
"Our navy was sad at first, then angry. I think we need this exercise," he said.
Associated Press writer Hyung-jin Kim contributed to this report from Seoul.