The incident prompted South Korea to step up border patrols, officials in Seoul revealed.
A defence ministry official said the defector told South Korean border guards he had shot his platoon and squad leaders before crossing the western side of the Demilitarized Zone around noon where he was taken into custody.
He also said the soldier used a loudspeaker to let South Korean guards know his intention to defect after the killings. The official said the motive behind the defection was unclear.
No unusual military movement was detected from the North Korean side of the border after the crossing, but South Korea immediately instructed its border troops to be on increased alert, a South Korean joint chiefs of staff official said.
There was no immediate comment from Communist North Korea’s state-run media.
Defections across the land border are rare, though North Koreans occasionally come to the South by boat.
Last year, a North Korean civilian defected to the South across the land border. The last defection across the Demilitarized Zone by a North Korean soldier occurred in 2010, officials said. Another soldier and an officer also defected to the South across the border in two separate crossings in 2008.
The vast majority of North Koreans fleeing their homeland travel through China and South-east Asia before arriving in the South.
More than 24,000 North Koreans have arrived in the South since a truce was called in the 1950-53 Korean War, which ended in a cease-fire. In the absence of a peace treaty, the two Koreas are still technically at war.
The area where yesterday’s defection took place is along the route to a South Korean-financed industrial complex in the North Korean border town of Kaesong, officials said.
Border security has been tighter than usual along the border in the past few years as military and political tensions between the rival Koreas soared, at a time of a leadership change in the north and pressure on the Communist regime from the West over its nuclear arms programme. In 2010, a South Korean naval ship sank and 46 of its sailors died in an incident blamed on North Korea, though Pyongyang denies involvement. Later that year, North Korea bombarded a South Korean front-line island, killing two marines and two civilians.