The breach occurred at around 12:30 p.m. (0330 GMT) on Sunday when the North Korean troops in the Demilitarised Zone (DMZ) separating the two Koreas crossed the military demarcation line, Seoul's Joint Chiefs of Staff (JCS) said on Tuesday.
JCS spokesman Colonel Lee Sung-jun said the military did not consider the breach to be intentional as the area was densely forested, obscuring border markings or any roads.
"They headed north immediately after our military's warning broadcasts and warning shots, and there were no unusual movements," he told a briefing.
Yonhap news agency, citing an unnamed JCS official, reported the troops were mostly carrying pick axes and other tools, and appeared to have become lost.
The incident came as the North has sent thousands of balloons containing trash in recent days to the South, including some 600 over the weekend, calling it a "gift" for North Korean defectors and South Korean campaigners who have flown balloons carrying anti-Pyongyang leaflets and aid parcels in the other direction over the border.
Seoul suspended a 2018 inter-Korean military pact and resumed military activities around the border, including reinstalling loudspeaker broadcasts, calling Pyongyang's action "base and dangerous."
The South Korean military has previously fired warning shots at North Korean soldiers crossing the border, but most such incidents took place around the maritime border which Pyongyang has disputed.
The two Koreas are still technically at war after their 1950-53 conflict ended in an armistice, not a peace treaty.
