The United States and South Korea launched war games Sunday as North Korea deployed missiles and threatened another attack if military exercises violate its territorial waters.
China said it would join efforts to ease the "worrisome" situation.
An artillery round was fired from North Korea on Sunday, officials said, triggering an evacuation order on Yeonpyeong Island, where four people died Tuesday in the North's bombardment. The order was lifted when no shells hit the tiny Yellow Sea home of fishing enclaves and military outposts.
South Koreans are demanding vengeance over the North Korean bombardment, but Pyongyang is blaming Seoul and the United States for triggering the attack.
The joint military drills began 6 a.m. local time (4 p.m. ET) in the Yellow Sea, west of the Korean peninsula town of Taean, south of the site of Tuesday's attack, according to Korean military officials quoted by Yonhap, Reuters and other news agencies.
However, officials told NBC News that the maneuvers would not start until midafternoon as the nuclear-powered aircraft carrier USS George Washington had just arrived in the area to join the exercise. They added that no live-fire exercises are planned during the three-day war games.
The George Washington, which carries 75 warplanes and has a crew of more than 6,000, will be accompanied by at least four other warships.
Anticipating 'provocation'China has expressed displeasure with the exercises while North Korea earlier said the consequences could not be predicted.
South Korean President Lee Myung-bak told ministers and aides to be ready for further "provocation" by North Korea during the drill.
"There is the possibility that North Korea may do some unexpected action, so please perfectly prepare against it through cooperation with the Korea-U.S. joint force," Lee was quoted by a spokesman as saying.
North Korea has placed surface-to-surface missiles on launch pads in the Yellow Sea, Yonhap news agency reported on Sunday. The agency said also that Pyongyang had moved surface-to-air missiles to frontline areas. The North's official KCNA news agency warned of retaliatory action if its territory was violated.
"We will deliver a brutal military blow on any provocation which violates our territorial waters," KCNA said.
The exercises are being held about 75 miles south of the disputed maritime border near Yeonpyeong Island.
"(The missiles) appear to be targeting our fighter jets that fly near the Northern Limit Line," a government source said on condition of anonymity, referring to the Yellow Sea border.
Earlier, North Korea worked to justify one of the worst attacks on South Korean territory since the 1950-53 Korean War. Four South Koreans, including two civilians, died after the North rained artillery on the small Yellow Sea island of Yeonpyeong, which is home to both fishing communities and military bases.
North Korea said civilians were used as a "human shield" around artillery positions and lashed out at what it called a "propaganda campaign" against Pyongyang.
It claimed the United States orchestrated last Tuesday's clash so that it could stage joint naval exercises in the Yellow Sea with the South that include a U.S. nuclear powered supercarrier — enraging the North and making neighboring China uneasy.
China in talksChina sent a senior official, State Councilor Dai Bingguo, to Seoul on Saturday for talks with Foreign Minister Kim Sung-hwan, South Korea's Yonhap news agency reported. Dai, accompanied by chief Chinese nuclear negotiator Wu Dawei, discussed Tuesday's attack and international talks on ending North Korea's nuclear programs, it said.
Lee told Dai: "We ask that China make a contribution to peace on the Korean peninsula by taking a more fair and responsible position on South-North Korea ties."
On Sunday, China's official Xinhua news agency said the talks between Dai and Lee were "in-depth and frank."
"Stressing that the current situation on the peninsula is worrisome, the two sides agreed that the parties concerned should make joint efforts to engage in serious contacts and dialogue to ease the tensions," Xinhua reported.
Also Sunday, Xinhua said that Choe Tae-Bok, chairman of North Korea's Supreme People's Assembly, or parliament, will visit China from Tuesday through Saturday.
New level of hostilityThe North Korean attack on an area with a civilian population marked a new level of hostility along the rivals' disputed sea border. Only eight months ago, according to the findings of a South Korean-led international investigation, a North Korean torpedo sank a South Korean warship in waters farther west, killing 46 sailors.
The aggression could be linked to the North's attempt to strengthen its government as it pursues a delicate transfer of power from leader Kim Jong Il to a young, unproven son. It also may reflect Pyongyang's frustration that it has been unable to force a resumption of stalled international talks on receiving aid in return for nuclear disarmament.
The attack laid bare weaknesses 60 years after the Korean War in South Korea's defenses against the North, which does not recognize the border drawn by the U.N. at the close of the conflict and which considers waters around Yeonpyeong as its territory.
The skirmish prompted Lee to replace his defense minister on Friday.
At a funeral Saturday near Seoul, South Korea's marine commander, Maj. Gen. You Nak-jun, vowed a "thousand-fold" retaliation for the attack. Dignitaries and relatives laid white flowers at an altar for the two marines killed in the North's attack. The mother of one of the victims fell forward in her chair in grief.
Passers-by paused at Seoul's main train station to watch funeral footage on a big screen.
"Once the enemy attacks us, it is our duty to respond even more strongly," said student Jeon Hyun-soo, 19. "The South Korean people want this."
'Let's go!'Elsewhere in Seoul, about 70 former special forces troops protested what they called the government's weak response and scuffled with riot police in front of the Defense Ministry, pummeling the riot troops' helmets with wooden stakes and spraying fire extinguishers.
"Let's go!" the activists shouted, as police, numbering several hundred, pushed back with shields.
North Korea's state news agency said that although "it is very regrettable, if it is true, that civilian casualties occurred on Yeonpyeong island, its responsibility lies in enemies' inhumane action of creating a 'human shield' by deploying civilians around artillery positions."
The North said its enemies are "now working hard to dramatize 'civilian casualties' as part of its propaganda campaign."
South Korea was conducting artillery drills Tuesday from the island, located just 7 miles from North Korea's mainland, but fired away from the mainland.
The North said it warned South Korea to halt the drills on the morning of the attack, as part of "superhuman efforts to prevent the clash to the last moment."
The North said that Sunday's U.S.-South Korean war games showed that the United States was "the arch criminal who deliberately planned the incident and wire-pulled it behind the scene."
The war games involving the USS George Washington supercarrier display resolve by Korean War allies Washington and Seoul to respond strongly to any future North Korean aggression. However, Washington has insisted the drills are routine and were planned well before last Tuesday's attack.
North Korea on Saturday warned of retaliatory attacks creating a "sea of fire" if its territory is violated.
President Lee told top officials "there is a possibility North Korea may take provocative actions during the (joint) exercise," and urged them to coordinate with U.S. forces to counter any such move, according to a spokesman in the president's office who spoke on condition of anonymity, citing official protocol.
Washington and Seoul have pressed China to use its influence on Pyongyang to ease tensions. China is impoverished North Korea's biggest benefactor and its only major ally.
The North's artillery barrage Tuesday destroyed civilian homes as well as military bases on Yeonpyeong Island.
President Lee has ordered reinforcements for the 4,000 troops on Yeonpyeong and four other Yellow Sea islands, as well as top-level weaponry and upgraded rules of engagement.
Most of the islanders fled to the mainland after the barrage set off fierce blazes that destroyed many of their communities. It will take six months to two years for island communities to rebuild, disaster relief official Kim Sang-ryul said.