South Korea's Prime Minister Chung Hong-won pledged strong retaliation against any North Korean attacks as Seoul on Sunday commemorated the fourth anniversary of the deadly shelling of a front-line South Korean island.

"The government and the military will preemptively block the possibility of North Korea's provocations through a water-tight defense posture, and if there is a provocation, we will sternly react to it," the prime minister said in a speech at a ceremony marking the anniversary of the 2010 attack on Yeonpyeong Island.

Defense Minister Han Min-koo delivered the speech in lieu of the prime minister who is currently on a trip to Egypt.

Sunday marks four years since the North's shelling attack on the island in the Yellow Sea. Two South Korean marines and two civilians were killed in the shelling, with 16 others wounded.

The attack is bitterly remembered by South Koreans as the first North Korean artillery attack on home soil since the 1950-53 Korean War.

"We should face the reality of our security situation. We should not forget that the freedom and peace we enjoy did not come without costs," the speech read.

"The security situation of the Korean Peninsula is very serious," the prime minister said, referring to the North's recent series of provocations in border areas. "In the face of unceasing provocations by the North, we should be firm in our defense posture more strongly than ever."

Chung also called on citizens not to forget the sacrifices of those who died while fighting for the country, saying that "no sacrifice is more honorable than those for the country and its people." 

Some 4,500 citizens, including the families of the victims, government officials and students from the island attended the ceremony held at the War Memorial Museum in downtown Seoul. (Yonhap)