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South Korean ships are shown off the coast of South Korea's Yeonpyeong Island on Sunday as the two countries prepared to begin war games. ((David Guttenfelder/Associated Press))

South Korea's Defence Ministry has ordered journalists to leave the front-line island of Yeonpyeong, citing high tension with North Korea.

The Defence Ministry says "the situation is not good" on the island, following the start of South Korea-U.S. military drills in Yellow Sea waters to the south.

Earlier Sunday, the military ordered islanders and journalists to evacuate to shelters after several artillery rounds were heard. None of the rounds hit the island.

As the United States and South Korea began their joint war games Sunday, South Koreans demanded vengeance over a deadly North Korean artillery bombardment that has raised fears of more clashes between the bitter rivals.

Meanwhile, the North 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 Tuesday when the North rained artillery onto 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 it.

The North said that Sunday's planned U.S.-South Korean war games showed that the U.S. was "the arch criminal who deliberately planned the incident and wire-pulled it behind the scene."

The war games, which involve the USS George Washington supercarrier, display resolve by 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.

The drills 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.

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

North Korea on Saturday warned of retaliatory attacks creating a "sea of fire" if its territory is violated.

The South Korean president told top officials "there is a possibility North Korea may take provocative actions during the [joint] exercise," and urged them to co-ordinate with U.S. forces to counter any such move, according to a spokesman in the president's office.