South Korean President Lee Myung-Bak on Thursday scolded his military for negligence after a North Korean soldier defector travelled unchecked across one of the world's most fortified borders.

The 22-year-old soldier defected on October 2, making his way through rows of electrified fencing dividing the two Koreas under cover of darkness.

Initially, military officials said he had been detected by surveillance cameras and guided to safety.

But inspectors later found the man turned himself in at the barracks of a frontline unit after his knock at the entrance of another unit went unanswered.

Lee called in Defence Minister Kim Kwan-Jin to scold the failed border watch and said the military had betrayed the public's trust, according to a presidential spokesman.

Lee demanded the military investigate the incident thorougly, "sternly punish those responsible" and take "fundamental steps" to watch the border more closely, the spokesman said.

South Korean media has described the incident as one of the biggest border security failures in years.

Jung Seung-Jo, chairman of the Joint Chiefs of Staff, said Thursday that the defector wandered around inside South Korea looking for a place to surrender, after slipping across the border undetected.

He showed up at the barracks of a frontline unit after he got no response when he knocked on the door at a nearby unit some 30 metres (99 feet) away, Jung told parliament.

"I feel sorry for causing confusion with the wrong account of what happened," Jung said.

The army division guarding the northeastern section of the border has already been placed under disciplinary inspection for failing to detect the crossing.

Three North Korean soldiers have defected across the heavily-mined border since August, including one who said he shot dead two superiors before crossing to the South on Saturday.

A worsening food shortage at barracks in the impoverished North has fanned frustration among its soldiers, Yonhap quoted a South Korean intelligence official as saying.