The incident happened shortly before sunset when North Korean soldiers fired two bullets toward a South Korean guard post in the eastern part of the Demilitarized Zone, said Maj. Kim Tae-hoon of the Joint Chiefs of Staff office.
South Korean soldiers immediately fired back six rounds, Kim said.
He said South Korea doesn't know what prompted the North troops to open fire and the secretive communist country hasn't said anything to the South about it.
The U.N. Military Armistice Commission, which supervises the cease-fire that ended the 1950-53 Korean War, will asked the North on Tuesday for an explanation of the incident.
The fighting ended in a cease-fire, not a peace treaty, meaning that the two Koreas are still technically in a state of conflict.