South Korea Wednesday marked the first anniversary of a deadly North Korean bombardment with sombre ceremonies and a military drill, as Pyongyang accused Seoul of provoking the attack.

Hundreds gathered at the national cemetery in the central city of Daejeon to commemorate two Marines killed when the North shelled Yeonpyeong island, near the tense Yellow Sea border, on November 23 last year.

The first attack on a civilian-populated area since the 1950-1953 war also left two civilians dead and destroyed or damaged scores of homes and other buildings.

The North launched 170 shells or rockets at the island, sparking outrage in the South and international alarm.

"How could we ever forget this day?" Prime Minister Kim Hwang-Sik said in a speech at the Daejeon event, attended by military commanders, politicians, Marines and families of the dead.

Tearful family members laid chrysanthemums in front of giant photos of the two. "The sacrifice for the nation will never be forgotten," read a large sign.

"It was a day of atrocity, in which North Korea launched an unrelenting shelling attack on our land for the first time since the Korean War," said Kim, vowing to "make no compromise" on further provocations.

The military staged a major land, sea and air exercise near the sea border Wednesday afternoon, simulating a response to a hypothetical new attack on the "frontline" islands.

The drill involved Marine artillery units permanently based on Yeonpyeong, jet fighters armed with ground-attack missiles and a 4,500-ton destroyer. The army was put on full alert.

The military, which faced severe criticism for its perceived slow and weak response last year, said the drill aimed to test the effectiveness of any future retaliation.

It has vowed to hit back hard, with artillery and air power, for any fresh attack.

Seoul has significantly strengthened troop levels and weaponry -- including multiple rocket launchers and Cobra attack helicopters -- on Yeonpyeong and other frontline islands in the past year.

Ceremonies were also held on Yeonpyeong near statues of the two Marines.

In Seoul, scores of North Korean defectors protested against the regime of their former homeland, calling leader Kim Jong-Il "devilish".

Last year's bombardment came eight months after the sinking of a South Korean warship with the loss of 46 lives.

The North denies the South's claim that it torpedoed the corvette and says the island attack was in response to a provocative South Korean artillery drill on Yeonpyeong.

Tense relations have eased slightly but Seoul insists Pyongyang admit responsibility for both incidents before there can be any substantial dialogue.

President Lee Myung-Bak, during a visit to the Marine Corps Command north of Seoul, called again for an apology.

"There has been no official apology from the North yet... I expect that the North will offer the message some day for the sake of reconciliation," Yonhap news agency quoted him as saying.

The North's main newspaper Wednesday reiterated that the South provoked the shelling and denounced it for "fabricating" threats to raise tension.

"Like we have said repeatedly, the Yeonpyeong incident was an outcome of the South warmongers' intentional provocation of war exercises," said the Rodong Sinmun.

Seoul and US officials said the North's bombardment last year was probably ordered to burnish the military credentials of Kim Jong-Un, youngest son and heir apparent of Kim Jong-Il.

The contested Yellow Sea border has long been a flashpoint and saw bloody naval clashes in 1999, 2002 and November 2009.