About 10 North Korean soldiers came near the MDL in Paju, some 40 km northwest of Seoul, a defence ministry official said.
The South Korean military fired about 20 rounds of warning shots with K-2 and K-3 rifles, after broadcasting warning, Xinhua reported.
This incident came after South Korea's army, navy and air force kicked off the largest-ever annual joint drill earlier in the day, which will run till Nov 21.
On Oct 19, the two Koreas exchanged fire across the land border in Paju.
At that time, the South Korean military launched warning shots at the North Korean troops who fired back against Seoul's guard post within the demilitarised zone (DMZ).
On Oct 18, some other North Korean soldiers approached the MDL in Cheorwon, 88 km northeast of Seoul, but they returned after the warning shots.