SEOUL - The Republic of Korea's President Lee Myung-bak paid a surprise visit Thursday to an island near the tense sea border with the Democratic People's Republic of Korea, the scene of deadly shelling two years ago.

His first-ever visit to Yeonpyeong Island is meant to boost the military's defense posture after the undetected defection by a DPRK soldier earlier this month prompted a public outcry over the border security lapse, a presidential spokesperson reportedly told local media.

The trip also came less than a week before the second anniversary of the DPRK's shelling on the front-line island, which killed two ROK's marines and two civilians.

The DPRK defended the shelling as an act of self-defense against a joint drill between the ROK and the United States near the western maritime border called Northern Limit Line (NLL).

The DPRK refuses to acknowledge the NLL, which was drawn unilaterally by the US-led United Nations Command at the end of the 1950-53 Korean War.

Critics say Lee's unprecedented trip, which came two months before the country elects his successor, was politically motivated.

Lee's conservative ruling Saenuri Party claims that the president's liberal predecessor, Roh Moo-hyun, once told late DPRK leader Kim Jong-iL in a closed-door meeting that he does not recognize the maritime border.

The allegations are seen as an attempt to delegitimize Moon Jae-in of the main opposition Democratic United Party, who served as chief of staff to Roh. Roh's former aides have flatly rejected the charge.