South Korea's defense chief apologized publicly and dismissed three officers Monday after his military failed to spot a North Korean soldier defecting to the South across their heavily guarded border.

No one detected the soldier — even though he scaled barbed wire fences on the South Korean side of the border — until he knocked on the door of front-line South Korean barracks to say he was defecting. The Oct. 2 incident raised serious questions about South Korea's military surveillance capabilities.

Defense Minister Kim Kwan-jin told a news conference he "deeply" apologizes for causing public concern over the defection.

"There were an obvious failure in security operations and faults in situation-reporting systems," Kim said.

A total of 14 officers, including five at the general level, will be investigated for possible punishment over the incident, ministry spokesman Kim Min-seok said. The ministry dismissed three officers, including a division commander, from their posts later Monday, ministry officials said.

Last week, President Lee Myung-bak ordered stern punishment for those responsible for the security breach.

A ministry statement said border security will be strengthened by deploying more guards and modern surveillance equipment and installing more wire fences.

The Korean Peninsula has remained divided along 4-kilometer-wide (the 2.5-mile-wide) Demilitarized Zone since the Korean War ended with an armistice, not a peace treaty, in 1953. It's guarded by hundreds of thousands of combat-ready troops on both sides and is strewn with land mines and laced with barbed wire.

About 24,000 North Koreans have defected to South Korea since the end of the Korean War, but defections across the land border rare with the vast majority of defectors fleeing through China and Southeast Asian nations for the South.

Defections by soldiers are unusual as well, although another North Korean soldier defected to South Korea via the land border after killing two of his superiors earlier this month.