South Korean President Lee Myung-bak made a surprise visit Thursday to an island near the tense border with North Korea that was shelled by Pyongyang two years ago.

Stressing the need to defend the maritime border "to the last man," Lee said the 2010 attack on Yeonpyeong island was an example of North Korea's tendency to launch isolated, provocative assaults out of the blue.

His visit and comments are likely to fuel cross-border tensions, already raised by a series of niggling maritime confrontations, defections and a new US-South Korean deal to nearly triple the range of the South's missile systems.

"The reason that we build up (the military) is not only to bolster our means to retaliate but to prevent the North's provocation," Lee was quoted as saying by Yonhap news agency.

"If North Korea provokes us, we have to retaliate strongly. We always have to remain vigilant," he said, as he inspected an anti-artillery radar unit and an artillery company on the island.

"If the North makes any (conciliatory) gesture, it is just a ruse and at such times we have to keep up our guard all the more," he added.

There are widespread concerns in the South that Pyongyang may try to instigate a military clash that would temporarily destabilize the Korean peninsula in the run up to South Korea's presidential election in December.

The North shelled Yeonpyeong - near the disputed maritime border in the Yellow Sea - on November 23, 2010, leaving two South Korean soldiers and two civilians dead.

The South retaliated with its own artillery bombardment on two targets in the North, triggering fears that the incident could provoke a wider conflict.

The de facto maritime boundary between the two Koreas, the Northern Limit Line, is not recognized by Pyongyang, which argues it was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War.

Lee's visit came on the day that South Korea announced an annual, large-scale military exercise aimed at countering threats from North Korea.

The week-long Hoguk exercise beginning October 25 will involve 240,000 army, navy, air force and marine corps personnel, along with police officers, a spokesman of the Joint Chiefs of Staff said.

"The exercise will feature drills against infiltration, regional provocations or an all-out war by North Korea," a defense ministry spokesman told reporters.

Cross-border ties have been generally icy since Seoul accused Pyongyang of torpedoing one of its warships in March 2010 with the loss of 46 lives, which the North denied.