Prime Minister Chung Hong-won stressed that the most important goal for the country at the moment is to maintain a strong defensive posture and that no allowances must be given to elements that try to weaken this resolve.

Speaking at a ceremony marking the third anniversary of the shelling of Yeonpyeong Island at the War Memorial of Korea in Seoul, the premier said that the building of a prosperous country is based on air-tight defense. He added that maintaining vigilance is the only way to repay the debt to those who sacrificed their lives in the defense of the country.

The North Korean attack, which marked the first time the communist country directly attacked any part of South Korean territory since the 1950-53 Korean War, caused the death of two marines and two civilians.

Seoul has held ceremonies to mark the incident to pay tribute to those killed and highlight the country's determination to prevent a reoccurrence of the tragedy.

"The shelling shows the critical state of South Korea's security environment as the only divided country on the face of the world," he said. Chung said people must realistically view the challenges facing the country and fully appreciate the peace we take for granted.

"The government will take a firm stance on any development that endangers the safety of the people and security of the country, and zero tolerance will be given to those elements that threaten basic principles of free democracy and security resolve," he said.

The remarks come as Seoul has accused some progressive groups of trying to overthrow the government to help the North.

On North Korea, the official said the country should desist from taking foolhardy military provocations and strive to ensure lasting peace on the Korean Peninsula.

After the speech, the prime minister paid his respects to the relatives of the two marines killed and stressed the country will never forget the sacrifices they made.

The ceremony was attended by 4,000 people, including soldiers and representatives from all parts of society.

Related to the third anniversary of the shelling the ruling Saenuri Party and the main opposition Democratic Party (DP) paid their respects to those killed in the attack and pledged to redouble their efforts to strengthen national defense.

Saenuri spokeswoman Min Hyun-joo, in particular, echoed Chung's call to root out groups that aim to weaken the country.

She said it is highly regretable that the mass given by left-leaning Catholic Priests for Justice in Gunsan on Friday gave the impression of legitimizing the North's attack on the island.

She said those involved in the mass should have shown respect for the dead.

Media reports said some at the mass that called for the resignation of President Park Geun-hye said the North had the right to attack if South Korea and the United States carried out war exercises near the sea demarcation line separating the two Koreas.

Some even raised doubts about Seoul's claims that the North sank the South Korean warship in March 2010 that left 46 sailors dead.

The DP, however, claimed that the former Lee Myung-bak administration failed to bolster national defense.

A deputy spokesperson warned that the incumbent Park Geun-hye administration is following the same foot steps as the previous government.

"The Park administration should consider the rapprochement policies pursued in the past," the politician said. (Yonhap News)