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.
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.
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"His organs are extremely damaged... we do not know how long he can hold up," Lee said, describing the soldier's condition as "very serious."
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 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.