Story highlights South Korean soldiers hear gunfire before the defector crosses

The North Korean says he killed his platoon and squad leaders

Defections over land via the heavily armed DMZ are rare

The number of refugees fleeing per year has climbed dramatically over the years

A North Korean soldier on guard duty at the border dividing the two Koreas defected, telling authorities who received him in the South that he killed his superiors before fleeing across the Military Demarcation Line.

South Korean soldiers at their guard posts reported hearing gunfire before he crossed into the South shortly after noon local time, according to a news release from South Korea's Joint Chiefs of Staff.

Soldiers in the South took the defector in on their side of the border and brought him to a safe place for questioning then tightened security in the area.

The North Korean said that "while he was on guard duty, he killed his platoon and squad leaders and defected thereafter," the JCS said.

Defections over land through the heavily armed and fortified Demilitarized Zone (DMZ) are rare, as most fleeing the communist North prefer the greater odds of success of crossing by sea.

"The last North Korean soldier who defected (over land) was in March 2010," said a spokesman from the defense ministry in Seoul.

But with his comrades dead and no longer capable of hindering him, it was easy for the soldier to flee south.

"The distance between the North Korean guard post and the South Korean guard post is 500 meters," the official said, who asked not to be named, because he is not authorized to speak with the media.

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, over two-thirds of them women.

The annual number of refugees has climbed steadily from less than 100 in the 1990s to more than 2,000 per year since 2006.