South Korea is holding a day of ceremonies and military drills on Wednesday to mark the one-year anniversary of a deadly North Korean shelling of an island just south of the contested Yellow Sea border.

Hundreds gathered at a national cemetery in the central city of Daejeon to remember two marines and two civilians who were killed when North Korea launched 170 rounds of artillery on the island of Yeonpyeong last November.

Prime Minister Kim Hwang-Sik promised a tough response to any further attacks by the North.

The incident was the first North Korean shelling of a civilian-populated South Korean territory since the Korean War in the 1950s.

Later Wednesday, South Korea will hold a major military exercise to simulate a response to any North Korean attack on the island.

The drill will involve artillery guns and fighter jets. The South Korean army will also be put on alert.

South Korea's government has been criticized by some for responding passively to the Yeonpyeong shelling. But Prime Minister Kim said Wednesday his government has increased troop levels and weaponry along the “frontline” islands in the past year.

Last year's shelling came only eight months after 46 seamen were killed in the sinking of a South Korean warship, which Seoul blamed on Pyongyang.

North Korea denies attacking the ship, and says the shelling of Yeongpyeong was in response to a “provocative” South Korean military exercise.

Though tensions have slowly eased, South Korea continues to insist that the North admit responsibility for both incidents before resuming any substantial dialogue.