A South Korean army soldier, center bottom, shakes hand with a KPA soldier before crossing the Military Demarcation Line inside the DMZ, December 12, 2018. (AP)

Seoul's military said on Friday that it fired warning shots after DPRK soldiers briefly crossed the heavily fortified border, marking the third such incident this month.

South Korea has previously stated that DPRK has been strengthening the border by constructing tactical roads and increasing the number of landmines, resulting in casualties among its troops due to accidental explosions.

On Thursday morning, "several North Korean soldiers who were working inside the DMZ on the central front line crossed the Military Demarcation Line," Seoul's Joint Chiefs of Staff said.

"After our military's warning broadcasts and warning shots, the North Korean soldiers retreated back northward," they added.

This is the third incident in less than two weeks where South Korean authorities accused DPRK troops of crossing the MDL. However, it was determined that the first incident was unintentional and that the troops who were carrying construction equipment had lost their way on June 9.

Again, the JCS said that around 20 to 30 DPRK soldiers carrying work tools crossed the MDL in the central sections of the border at around 8:30 am (local time) on Tuesday this week.

DPRK constructing infrastructure in the DMZ amid rising tensions

Earlier this week, the South Korean Yonhap News Agency reported that the DPRK is constructing various infrastructure in the DMZ, a zone that is 250 kilometers long and approximately four kilometers wide, which splits the two Koreas.

The activities are taking place north of the MDL, according to Yonhap, which cited an unnamed military source.

"Recently, the North Korean (DPRK's) military has been erecting walls, digging the ground, and constructing roads in some areas between the Military Demarcation Line (MDL) and the Northern Limit Line in the DMZ," the military source told the South Korean news agency.

So far, the South Korean military said that it was "closely tracking and monitoring the activities of the North Korean military," stressing that "further analysis is required," as the military does not know what is being built north of the MDL.

The DMZ has a long history of controversies from both sides and has been a hotspot for nuisances. The two Koreas have sought to creatively disrupt and provoke each other via various methods, including the deployment of loudspeakers that play K-Pop and flying hot balloons that carry garbage and excrement over to the other side.

Behind these provocations lies a much deeper division between the two governments that stems back to the 1950s Korean War, in which the United States and its allies supported the South Korean government against what they perceived as DPRK's growing influence and advance across the Korean peninsula.

Attempts to soothe ties between the two Koreas have ended in failure as the US and South Korea take a more aggressive approach against the DPRK, launching wide-scale military exercises throughout the past two years, while Seoul continues to threaten Pyongyang with military action.