Story highlights The North Korea-South Korea border is considered the most heavily militarized in the world

This month, a North Korean soldier crossed it undetected

He is one of three defectors in the past two months, a lawmaker says

The South Korean defense minister apologizes for the security failure

South Korea on Monday reprimanded five senior military officials after admitting that a North Korean soldier had managed to cross the heavily patrolled border between the two countries undetected and knock on the door of his enemy's barracks.

Revelations about the ease with which the North Korean soldier reached the South Korean side of the border, considered to be the most heavily militarized in the world, have prompted criticism of the nation's military.

The soldier, whose name and rank were not disclosed, jumped three barbed-wire fences along the eastern part of the border without being spotted on October 2, according to the South Korean Defense Ministry.

He was picked up by South Korean soldiers in the province of Kangwon only after he knocked on a barracks door, wishing to defect, the ministry said. He had escaped from his unit and traveled about 50 kilometers (31 miles) through North Korean territory to get to the border zone.

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South Korean Defense Minister Kim Kwan-jin made a public apology in a live broadcast on national television Monday, calling the incident "a clear failure in security operations."

"As the defense minister, I express sincere apology for causing concern to our people," he said.

Kim said a lieutenant general, two major generals and two brigadier generals were facing disciplinary action. He also pledged to enhance security measures and increase the number of soldiers around the border area.

The security breach was deemed "lamentable" in an editorial last week by The Korea Times, an English-language newspaper.

"What if the defector were an agent armed with guns and grenades who sneaked into the barracks on orders from the North?" the editorial asked.

It also criticized the initially misleading information about the situation from defense officials.

The South Korean military had announced a subsequent defection across the border on October 6, in which a North Korean solider on guard duty at the border told authorities in the South that he had killed his superiors before fleeing across the Military Demarcation Line.

At the time, defense officials said it was the first defection over land since March 2010.

But news of the controversial October 2 defection and another in September later emerged after a South Korean lawmaker, Kim Kwang-jin from the main opposition Democratic United Party, questioned officials from the Joint Chiefs of Staff in a parliamentary hearing last week.

In a statement, he criticized the "lies and deception from the military."

According to the lawmaker, at least 10 divisions from the South Korean army and two Marine Corps divisions are guarding the roughly 250-kilometer-long border area.

Defections over land through the heavily armed and patrolled border area are rare, as most prefer to try crossing by sea or going through China.

More than 24,000 refugees have entered the South from the North since the country's division, according to the Ministry of Unification in Seoul.