The rare exchange of fire took place a fortnight before the leaders of the world's 20 top economies meet for a G20 summit in the South Korean capital Seoul, about 100 km south of the demilitarized zone.
The South's Defense Ministry said in a statement none of its soldiers was hurt, and there had been "no more unusual activity by the North". A South Korean military official said the army had been put on heightened alert.
It was not immediately clear what was behind the skirmish, but in the past the North has carried out similar provocations around the time the South has hosted prominent international events.
YTN television said, however, it was unlikely that the North had deliberately fired across the border toward the South only hours before families separated by the 1950-53 Korean War were due to be reunited for the first time.
The North Korean front-line guard post fired two shots toward a South Korean guard post across the DMZ and the South returned fire with three shots, a joint chiefs of staff official said.
The South Korea military official said he had not received any communication from the North. A United Nations team will be sent to the area on Saturday, he added. The North Korean shots were fired at a front-line unit in Cheorwon in the eastern province of Gangwon.
