The South Korean military conducted a large-scale exercise Wednesday on its tense maritime border with North Korea in a show of force to mark the one-year anniversary of the North's deadly shelling of Yeonpyeong island.

Helicopters, fighter jets and artillery were used in the drill, which was meant to simulate a response to a North Korean attack on one of the South's so-called “front-line” islands.

During a ceremony commemorating the four people killed in the November 2010 shelling, South Korean Prime Minister Kim Hwang-Sik promised a tough response to any further attacks.

South Korea's government has been criticized for its response to the Yeonpyeong shelling. Prime Minister Kim said his government has increased troop levels and weaponry in order to prevent future attacks on the disputed Yellow Sea border.

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

South Korea continues to insist that the North apologize for the attack before resuming substantial dialogue.

South Korean President Lee Myung-bak said Wednesday that he expects Pyongyang to issue an official apology “for the sake of national dialogue.”

Pyongyang blames the South for the shelling, saying it was in response to a “provocative” South Korean military drill on the island.