South Korea's president has rebuked the military for its failure to spot a North Korean soldier before he crossed the two countries' heavily armed border last week.

Officials say the soldier knocked on the door of a South Korean barracks near the border last Tuesday and told troops there he was defecting.

The incident has prompted criticism of the military's surveillance capabilities.

President Lee Myung-bak summoned the defense minister Thursday and said those responsible for the failure should be punished.

Officials say the front-line unit also falsely claimed that it had spotted the North Korean on a surveillance camera.

On Saturday, another North Korean soldier defected across the border after killing two superiors. Such crossings are unusual.

The Koreas remain technically at war after their 1950-53 conflict ended in a truce.