Story highlights Lee Myung-bak makes his first visit as president to view his troops' military readiness

He is accompanied by his defense minister

North Korea shelled the island in November 2010

South Korean President Lee Myung-bak vowed Thursday that his nation would protect its sea border "to the death" as he visited a nearby island shelled by North Korea two years ago.

Lee made his first visit as president to view his troops' military readiness on the frontline island.

"Now that I am here, I can feel the confrontation (with the north)," he said. "I realize the importance of Yeonpyeong island."

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He was accompanied by Defense Minister Kim Kwan-jin.

North Korea shelled the island in the Yellow Sea in November 2010, killing two marines and two civilians.

Pyongyang claims it was responding to South Korea holding a military drill nearby.

The island is close to the maritime border, the Northern Limit Line, which Pyongyang does not recognize.

During his three-hour visit, Lee reiterated his stance.

"I believe that all Korean marines should know that we have to protect the NLL to the death," he said.

The South Korean president said his military would have no choice but to launch a counterattack if there was another provocation.

Last month, North Korean leader Kim Jong Un visited the North Korean islands from which his troops had fired on Yeonpyeong. He told the military to be vigilant and ready to lead a "sacred war".

These disputed waters have seen deadly skirmishes in recent years.

Last month, North Korean fishing boats strayed south of the limit line. The South Korean joint chiefs of staff said they fired warning shots to ensure the boats returned North.