None of the rounds landed on Yeonpyeong Island, military officials said, but the incident showed how tense and uncertain the situation remains along the Koreas' disputed maritime border five days after a North Korean artillery attack decimated parts of the island and killed four South Koreans.

As the rhetoric from North Korea escalated, with new warnings of a "merciless" assault if further provoked, a top Chinese official made a last-minute visit to Seoul to confer with South Korean President Lee Myung-bak.

Lee and State Councilor Dai Bingguo, a senior foreign policy adviser, discussed the North Korean attack and how to ease the tensions, according to Lee's office. Dai also met with South Korean Foreign Minister Kim Sung-hwan on Saturday, the Foreign Ministry said.

Meanwhile, the chairman of North Korea's Supreme People's Assembly, Choe Thae Bok, was due to visit Beijing starting Tuesday, China's official Xinhua News Agency said.

Washington and Seoul have urged China, North Korea's main ally and biggest benefactor, to step in to defuse the situation amid fears of all-out war.

The Korean peninsula remains in a technical state of war because the 1950-53 war ended in a truce, not a peace treaty. Their border is one of the world's most heavily fortified, guarded by troops on both sides.

However, North Korea disputes the maritime border drawn by U.N. forces at the close of the war, and considers the waters around Yeonpyeong Island — 50 miles (80 kilometers) from the South Korean port of Incheon but just 7 miles (11 kilometers) from the North Korean mainland — its territory.

The Koreas have fought three bloody naval skirmishes in the waters since 1999, as recently as a year ago. And eight months ago, a South Korean warship, which had been involved in one of those skirmishes, went down in an explosion, killing 46 sailors.

An international team of investigators concluded that a North Korean torpedo sank the ship. The two Koreas have remained locked in a standoff over that incident, with South Korea demanding a show of regret for the attack and North Korea denying any involvement.

Tuesday's attack — on an island with a civilian population of 1,300 — marked a new level of hostility along the rivals' disputed sea border. Two marines and two civilians were killed when the North rained artillery on Yeonpyeong Island in one of the worst assaults on South Korean territory since the Korean War.

The attack took place as North Korea carries out a delicate transfer of power from leader Kim Jong Il to a young, unproven son in what many see as the heir's bid to win the military's loyalty. 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 also laid bare weaknesses in South Korea's defenses against North Korea.

North Korea said Saturday that civilian deaths were "regrettable," but blamed South Korea for staging military drills in the waters against Pyongyang's warnings that it would consider such exercises a provocation.

Meanwhile, North Korea mounted surface-to-air SA-2 missiles on launch pads on a west coast base and aimed at South Koreean fighter jets flying near the western sea border, the Yonhap news agency reported, citing an unidentified South Korean government source.

South Korea's military said it couldn't confirm the deployments. An official at the Joint Chiefs of Staff said the North already deploys anti-ship missiles on its west coast bases.

The previously planned joint war games that U.S. and South Korea launched Sunday were sure to heighten the tensions.

Washington insists that the drills involving the nuclear-powered USS George Washington supercarrier are routine and were planned well before last Tuesday's attack.

The exercises kicked off Sunday morning when ships from both countries entered the exercise zone, an official with South Korea's joint chiefs of staff said on condition of anonymity, citing office rules.

However, a spokesman for the U.S. military in South Korea said U.S. ships were still steaming toward the area and that the drills would not officially begin until later in the day.

North Korea has expressed outrage over the Yellow Sea drills involving a U.S. nuclear-powered supercarrier, and issued a fresh warning Sunday.

"We will launch merciless counter-military strikes against any provocative moves that infringe upon our country's territorial waters," the North's main Rodong Sinmun newspaper said in an editorial carried by the official Korean Central News Agency.

Sunday's burst of artillery fire in North Korea appeared to be the second in as many days.

Officials were investigating the exact location of Sunday's artillery fire, an official with South Korea's Joint Chiefs of Staff said. He spoke on condition of anonymity, citing office rules.

North Korea also fired artillery fire Friday on its land just as the U.S. military's top commander in the region, Gen. Walter Sharp, was touring Yeonpyeong Island.

Tuesday's attack reduced dozens of homes on the island to rubble. All but a handful of residents have evacuated to the mainland, officials said.