SEOUL -?The Republic?of?Korea's defense minister publicly apologized Monday over the security lapse at the border with the Democratic People's Republic of Korea (DPRK) following revelations a DPRK defector crossed the heavily fortified land border undetected.

Contrary to the military's initial account of the incident, inspectors later found out that the 22-year-old DPRK soldier, who said he killed his superior before crossing the Military Demarcation Line on Oct 2, was not spotted until he knocked on the door of a barracks and surrendered.

The security vacuum and the attempted cover-up drew harsh criticism from the public wary of lingering cross-border tensions after two deadly border incidents in 2010 killed 50 South Koreans.

"I sincerely apologize for causing public concern," Defense Minister Kim Kwan-jin said at a press conference in Seoul, acknowledging a "surveillance failure".

A total of 14 high-ranking military officials, including five generals, will face disciplinary measures, the minister said, pledging to beef up security along the border and reinforce border fences "as soon as possible".

The two Koreas remain technically at war with each other after the 1950-53 Korean War ended with a truce instead of a peace treaty.

Some 24,000 people have fled the DPRK and defected to the South since the division of the Korean peninsula, according to government data.