North Korean soldiers briefly crossed the heavily-fortified military demarcation line with South Korea in the second such incident between the two hostile countries in less than two weeks, Seoul said on Tuesday.
After South Korean border patrols fired warning shots, the North Korean soldiers immediately retreated across the border again, said the Joint Chiefs of Staff in Seoul.
It comes after South Korea reported a similar incident last week said to have occurred on June 9.
As in that earlier incident, the group of North Korean soldiers, comprising around 20 to 30 forces, carried construction equipment, according to the South Korean military.
They probably crossed the demarcation line that lies within a demilitarized buffer zone accidentally, the Joint Chiefs of Staff said, adding that no unusual activity had been observed.
North Korea has recently sent soldiers to various locations in the northern part of the demilitarized zone (DMZ) to carry out construction work.
South Korea's General Staff said that, in addition to laying landmines, the North Koreans also appear to have reinforced roads used for military purposes and erected structures whose purpose remained initially unclear, but might potentially be anti-tank obstacles.
Several North Korean soldiers have been killed by landmine explosions since April, according to the South Korean military.
Tensions have been on the rise again along the DMZ and the maritime border that separate the two Korean nations.
Under international law, the Koreas are still at war since the end of their conflict in 1953, since there never was a peace treaty.
The buffer zone, which is around 240 kilometres long and 4 kilometres wide, was established at the end of the fighting and forms the current de facto border.