The US-led United Nations Command (UNC), which monitors the Panmunjom border truce village where the defection occurred Monday, said the soldier had driven close to the heavily-guarded, military demarcation line separating the two Koreas.
"He then exited the vehicle and continued fleeing south across the line as he was fired upon by other soldiers from North Korea," the UNC said in a statement.
An official with the South Korean Joint Chiefs of Staff said the North's border guards fired at least 40 rounds.
A doctor treating the soldier -- who was airlifted to a hospital for emergency surgery -- said he had been shot half-a-dozen times and sustained a serious stomach injury.
"He has at least six gunshot wounds on his body and the penetrating wound in the abdomen is the most serious", Lee Cook-Jong told reporters.
It is very rare for the North's troops to defect at the truce village, a major tourist attraction bisected by the demarcation line and the only part of the frontier where forces from the two sides come face-to-face.
The 1953 ceasefire ending Korean War hostilities was signed at Panmunjom, and it has since hosted numerous rounds of inter-Korean talks -- sometimes held in huts that straddle both sides of the border line.
Previous defections at Panmunjom
The fact that the defector drove to the frontier suggests he may not have been a member of the elite corps of North Korean troops posted to Panmunjom, who are carefully vetted and selected for their loyalty.
No tourists were present at the time, because tours do not run on Mondays.
According to the South Korean military there was no exchange of fire across the border, and the UNC statement stressed that no South Korean or US forces were harmed.
Unlike the rest of the frontier, Panmunjom is not fortified with minefields and barbed wire and the border is marked only by a low concrete divider.
After racing across the frontier, the soldier took cover near a building on the South side.
The Joint Chiefs of Staff official said he was found collapsed in a pile of fallen leaves and recovered by three South Korean soldiers crawling on their stomachs to his position.
There have been previous defections at Panmunjom, most notably in 1984 when Vasily Yakovlevich Matuzok -- an elite student from Moscow who was being groomed to become a Soviet diplomat -- sprinted across the border and triggered a 30-minute gun battle that left four dead.
Visiting the border village with a delegation, Matuzok asked a colleague to take his picture, backed closer to the demarcation line and then suddenly turned and made a run for it.
North Korean guards immediately drew their weapons and set off in pursuit. The moment they crossed the line, a shooting match erupted and Matuzok was forgotten as the rival troops engaged on the South side of the border.
It was the greatest loss of life to occur in what is technically called the Joint Security Area.
Another gun battle was recorded in 1967 when a senior journalist from the North's state-run KCNA news agency crossed the border while covering military talks underway in Panmunjom.
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