South Korea says several North Korean soldiers briefly crossed the heavily-fortified military demarcation line between the two hostile countries in an incident over the weekend.
After South Korean border patrols fired warning shots, the North Korean soldiers immediately retreated across the border again, with no "unusual activity" reported from the Sunday incident, the Joint Chiefs of Staff in Seoul said on Tuesday.
According to South Korean news agency Yonhap, citing the general staff, some 20 North Korean soldiers had temporarily entered South Korea.
The South Korean military said it assumes that the soldiers crossed the border, which lies inside a demilitarized buffer zone, unintentionally while conducting an unidentified task within the zone.
However, the Chiefs of Staff stressed the military continued to closely monitor any activity on the other side of the border.
Tensions have been on the rise again along the demilitarized zone (DMZ) and the maritime border that separate the two Korean nations, who are still officially at war since a conflict in the 1950s.
Last weekend, North Korea once again sent numerous balloons with plastic bags full of rubbish across the border, according to Seoul.
North Korea's balloon actions are a reaction to similar activities by South Korean groups, which repeatedly send thousands of leaflets and other propaganda material across the border in huge gas balloons.
In light of recent tensions, South Korea's government recently decided to suspend a 2018 military agreement with North Korea on confidence-building measures at the border.