By Kim Young-jin

President Lee Myung-bak said Friday that he will ensure an “iron tight” defense against North Korean provocations, as he joined surviving family members to mourn on the 10th anniversary of a bloody naval clash near the disputed West Sea border.

Marking the first time a sitting president has participated in the annual ceremony, Lee call the incident ­ which left six South Korean sailors dead and 18 injured -- a premeditated provocation in line with recent attacks.

“I will ensure an iron-tight defense of the Republic of Korea,” he said in a speech at the Navy’s 2nd Fleet Command in Pyeongtaek, Gyeonggi Province, adding that the military would respond “sternly to any provocations and boldly counter aggression to defend the nation.

“The country will never forget (the fallen sailors). They put themselves on the line to defend the country.”

The solemn ceremony ­ attended by thousands of sailors, officials and family members - underscored grave military tensions in the West Sea.

As the 2002 World Cup hosted by Korean and Japan was drawing to a close, two North Korean patrol ships crossed the Northern Limit Line (NLL) engaging two South Korean vessels. Pyongyang, which disputed the NLL, is believed to have sustained 30 deaths or injuries in the incident.

The incident was a particular surprise given its timing during a period of rapprochement spearheaded by late former President Kim Dae-jung.

In 1999, a North Korean patrol boat violated the NLL by an estimated 10 kilometers but was driven back by the South in a gunfight that killed several North Koreans. A similar event occurred in 2009 as well.

The West Sea became a flashpoint again in March 2010, when Pyongyang sank the Cheonan warship, killing 46, though it still denies this action. Eight months later it shelled Yeonpyeong Island in response to routine military drills by the South, killing four citizens and causing tensions to rise to their worst point in decades. Many tied the attacks to new leader Kim Jong-un, saying the then-heir apparent was consolidating his military credentials.

Lee reiterated his stance that Seoul is willing to engage Pyongyang but only when it approaches in a sincere manner.

“We are ready to cooperate with North Korea for the sake of peace, coexistence and co-prosperity,” he said, urging Pyongyang to change its “Cold War mindset.”

Following a decade of rapprochement, tensions have reached boiling point during Lee’s administration, which has prioritized human rights of citizens in the North and tied provision of large-scale assistance to denuclearization. Ties between the two nations have continued to be sour under new leader Kim, whose regime continues to sling vitriolic rhetoric at Seoul.

Drawn up by the U.S.-led United Nations Command at the end of the 1950-53 Korean War, the NLL has served as the de facto sea border. Pyongyang calls for a redrawing of the border further South, but Seoul has maintained its stance on the issue that such a move would be a territorial concession.

Some say that further provocations remain possible because Kim might seek to consolidate support after being handed power following the death of his father Kim Jong-il six months ago. Such a prospect has raised concern because Seoul altered its rules on engagement following the 2010 provocations and has vowed stern retaliation.