"The drills started around 6 a.m. Sunday with the USS George Washington joining our warships in the Yellow Sea," an official with South Korea's Joint Chiefs of Staff said. However, the exercises are taking place off the west coast town of Taean, far south of the tense Yellow Sea border and Yeonpyeong Island.
"The intensity for the Yellow Sea drills will be higher than planned," another military official said. "Participating troops will conduct live-fire shooting and bombing drills."
The nuclear-powered USS George Washington, with more than 6,000 sailors and 75 fighter jets, is among the 10 warships taking part in the exercises that will run until Wednesday.
Other U.S. ships include the USS Cowpens, USS Shiloh and the USS Stethem. South Korea has deployed a 7,600-ton Aegis destroyer, two 4,500-ton destroyers, frigates and anti-submarine aircraft, JCS officials said.
Tensions rose dramatically between the two Koreas last week when the North unexpectedly shelled Yeonpyeong Island, killing two South Korean marines and two civilians. The South returned fire with 155mm K-9 self-propelled howitzers.
South Korean television carried recordings of the daylight shelling. Civilians are seen running for cover as several explosions are seen and heard amid buildings and smoke rises in the near distance. The shelling damaged dozens of houses as well as some military buildings.
The shelling is believed to have been in response to South Korean military exercises in the politically sensitive sea area where the main Yeonpyeong Island is less than 8 miles from the North Korean mainland.
Yeonpyeong lies near the Northern Limit Line, the sea boundary agreed to by both Koreas in the 1953 Armistice that ended three years war. But North Korea increasingly has contested the agreement in the past 15 years.
The South Korean military has issued a warning to journalists on the island to leave as soon as possible.
At the funeral of the two marines in Seongnam, near Seoul, the military issued another warning, saying North Korea could face retaliation, although this was left unspecified.
Around 600 mourners including South Korean Prime Minister Kim Hwang-sik and the marine commander Maj. Gen. You Nak-jun, attended the funeral at a military hospital.
Chrysanthemums, the traditional flower of mourning in South Korea, were placed in front of photographs of the two marines, who were promoted posthumously and awarded medals.
"Our marine corps ... will carry out a hundred- or thousand-fold," retaliation against North Korea for launching Tuesday's attack, You said.
The North Korean attack also claimed a South Korean political victim. Defense Minister Kim Tae-young resigned over assertions that the military was caught flat-footed and then was slow to respond to the North's aggression.
South Korean President Lee Myung-bak accepted the minister's resignation "to improve the atmosphere in the military and to handle the series of incidents," a presidential official said.
Lee also is believed to have ordered reinforcements for the 4,000 troops on Yeonpyeong and four other Yellow Sea islands, as well as a revamp of South Korea's rules of engagement when attacked by the North.
North Korean media have reported their government's condemnation of the joint South Korean and U.S. military exercises, saying Pyongyang will deal "a merciless military counterattack" at any aggression aimed at its territory or attempts to enter its national waters. It would be "a legitimate exercise of the right to self-defense taken to defend the dignity and sovereignty of the DPRK."
The North Korean shelling sparked a flurry of diplomatic activity between China, one of North Korea's closest allies, and the United States, a main ally of South Korea.
"The pressing task now is to put the situation under control and prevent a recurrence of similar incidents," Chinese Foreign Minister Yang Jiechi reportedly told U.S. Secretary of State Hillary Clinton by phone, according to the ministry's Web site.