South Korean soldiers fired warning shots after North Korean troops crossed the border by mistake, Seoul’s military said on Tuesday.
The incident at the Demilitarised Zone (DMZ) on Sunday comes as tensions continue to rise between the two Koreas.
A small group of North Korean soldiers carrying field tools including pickaxes entered South Korea at 12:30 local time (05:30 GMT), Seoul’s military said. They were among 20 who were in the border area at that time.
They retreated immediately after the South Koreans fired the warning shots.
In recent weeks, the North has flown hundreds of rubbish-filled balloons to border towns in the South.
Seoul has responded by broadcasting propaganda and K-pop music to the North using loudspeakers. Activists have also flown propaganda balloons into the North.
There was no notable movement from the North in the DMZ after its troops retreated on Sunday, Seoul’s military said.
“Inside [the border area] the vegetation is overgrown, and the border markers are hidden. There are no paths, and they were wading through the overgrowth,” it said.
On Monday, North Korean leader Kim Jong Un’s sister, Kim Yo Jong, threatened the South with “new counteractions” if it continues loudspeaker broadcasts and does not stop activists from sending balloons.
Last December, Mr Kim ended all efforts at a peaceful unification with the South, accusing Seoul of “hostility” towards the North.
Since then, the North demolished a highly symbolic unification monument in Pyongyang and ended all communication with the South.
Earlier this month, South Korea suspended what remained of its 2018 military agreement with the North, which will allow it to resume drills and propaganda activities such as loudspeaker broadcasts.
South Korea had partially suspended the agreement last November, following the North’s launch of a spy satellite.
In recent months, Seoul detected North Korean soldiers planting landmines along the border and disconnecting railways to the South. North Korean soldiers were also seen installing guard posts within the DMZ.