Incident is the third this month and took place as Russian President Vladimir Putin was visiting Pyongyang.
South Korea’s military has said it fired warning shots after several North Korean soldiers briefly crossed the border, the third such incident this month.
The Joint Chiefs of Staff (JCS) said the incident occurred on Thursday when the North Korean soldiers crossed the Military Demarcation Line running through the middle of the Demilitarized Zone (DMZ) at about 11am (02:00 GMT).
“After our military’s warning broadcasts and warning shots, the North Korean soldiers retreated back northwards,” the JCS said on Friday.
Similar incidents took place on Tuesday and on June 9. On both occasions, which Seoul said appeared accidental, the North Koreans retreated quickly after warning shots were fired.
Nuclear-armed North Korea has deployed more soldiers and equipment to the border since abandoning a 2018 military agreement when South Korea suspended parts of the deal in response to Pyongyang’s successful launch of its first military spy satellite.
According to Seoul, they have been expanding tactical roads and laying more landmines, which have led to “casualties” as a result of accidental explosions.
The incident took place as Russian President Vladimir Putin visited Pyongyang for the first time in 24 years and the two countries signed a mutual defence pact that has raised alarm in Seoul, Tokyo and Washington.
On Friday, South Korea’s Foreign Minister Cho Tae-yul and United States Secretary of State Antony Blinken condemned the treaty as a serious threat to the peace and stability of the region.
Yonhap news agency reported Seoul would also summon Russian ambassador Georgy Zinoviev to lodge an official protest over the pact, which states that if either of the two is invaded or put in a state of war, the other side will provide military and other assistance “with all means” at its disposal and “without delay”.