YEONPYEONG ISLAND, South Korea – South Korea's government ordered journalists to leave a front-line island Sunday, citing tensions with North Korea, hours after the U.S. and South Korea launched a round of war games in Korean waters.

The South Korean Defense minister said journalists must leave because the "situation is not good" on Yeonpyeong Island, which was targeted last week by a deadly North Korean barrage.

Earlier Sunday, the sound of new artillery fire from North Korea sent residents and journalists on the front-line island scrambling for cover.

None of the rounds landed on the 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. Washington and Seoul have urged China, North Korea's main ally and benefactor, to help defuse the situation amid fears of all-out war. Beijing has called for restraint on all sides.

Lee pressed State Councilor Dai Bingguo, a senior foreign policy adviser, to contribute to peace in a "more objective, responsible" matter, and warned that Seoul would respond "strongly" to any further provocation, his office said in a statement.

Dai forwarded Beijing's condolences and pledged China's help in preventing tensions from worsening, Lee's office 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.

The border between North and South Korea is among the world's most heavily fortified, with the peninsula still technically in a state of war because the 1950-53 war ended with a truce, not a peace treaty.

North Korea also 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 area has seen several bloody skirmishes, including the sinking of a South Korean warship eight months ago, killing 46 sailors. An international team of investigators concluded that a North Korean torpedo sank the ship, but Pyongyang denies any involvement.

Tuesday's attack on the island, which has military bases as well as a civilian population of 1,300 who mostly make their living from fishing, marked a new level of hostility. Two marines and two civilians were killed, and 18 others wounded, when the North rained artillery on Yeonpyeong in one of the worst assaults since the Korean War.

North Korea said Saturday the civilian deaths were "regrettable," but blamed South Korea for staging military drills against Pyongyang's warnings that it would consider such exercises a provocation. Pyongyang accused Seoul of using Yeonpyeong's residents as human shields.

The North Korea military also has mounted conventional, surface-to-air SA-2 missiles on launch pads on a west coast base, aiming them at South Korean 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 had already deployed anti-ship missiles on its west coast bases.

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

Ships from both countries entered the exercise zone Sunday, an official with South Korea's joint chiefs of staff said on condition of anonymity, citing office rules.

Washington, which keeps 28,500 troops in South Korea to protect the ally, insists the drills involving the USS George Washington supercarrier were routine and planned well before last Tuesday's attack. However, North Korea expressed outrage over the Yellow Sea drills.

"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 was the second in three days. Authorities briefly ordered residents to evacuate, before recalling the order.

"We got the report that North Korea's artillery batteries were in the 'ready-to fire' posture," police chief Choi Du-gyu said. "So we decided to order residents to evacuate to keep them safe."

North Korea also staged an apparently artillery drill Friday, the guns sounding just as the U.S. military's top commander in the region, Gen. Walter Sharp, was touring Yeonpyeong Island. No shells landed anywhere in South Korean territory.

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

As monks chanted their morning prayers at Jogye Temple, Shim Jeong-wook, 74, said he didn't think North Korea would attack again, not with a U.S. aircraft carrier group in South Korean waters.

"I don't think North Korea will provoke while the U.S. Navy fleet is in the Yellow Sea," he said. "But who knows what will happen when it leaves?"

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Jean H. Lee reported from Seoul. AP writers Hyung-jin Kim and Kelly Olsen in Seoul, Christopher Bodeen and Gillian Wong in Beijing and Pauline Jelinek in Washington contributed to this report.