It said there was no exchange of gunfire between the two sides in 250-kilometer-long, 4-km-wide DMZ, which is fortified with land mines, fences and guard posts.
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 U.S. forces were harmed.
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.
"In the afternoon today, a North Korean soldier defected from a guard post on the North Korean side of the JSA toward our side and our military took him", the Yonhap news agency quoted an unidentified military official as saying.
These were the first gunshots fired at the truce village of Panmunjom in 33 years and the first defection of a North Korean soldier since 2007. 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.
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"Our military maintains full readiness while strengthening the alert in preparation for the possibility of provocations by the North Korean army", the JCS said. South Korean troops didn't fire at the North, he said.
More than 30,000 North Koreans have defected to the South since the end of the 1950-53 war, but it's rare for soldiers to flee across the DMZ, much less the JSA.
North Korea has typically accused South Korea of enticing its citizens to defect, something the South denies. They stand only several yards away from tall South Korean soldiers wearing aviator sunglasses and standing motionless like statues. The attack prompted Washington to fly nuclear-capable B-52 bombers toward the DMZ in an attempt to intimidate North Korea.
United Nations Command (UNC) said the soldier had driven close to the heavily-guarded, military demarcation line in a vehicle. The incident left three North Korean soldiers and one South Korean soldier dead.
The Associated Press contribute to this report.