North Korean soldiers build a strongpoint in the Demilitarized Zone, in this file photo taken from a South Korean observation tower in Paju, 37 kilometers northwest of Seoul, on June 4. (Yonhap)

North Korea's military has been carrying out unexplained construction activities inside the Demilitarized Zone (DMZ) separating the two Koreas, according to a military source Saturday.

"Recently, the North Korean 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 source said.

The source added it was unclear whether these activities indicate an intention to build a long wall north of the MDL or simply to establish defensive structures at specific points.

Earlier this week, about 20 North Korean soldiers briefly crossed the inter-Korean land border before going back to the North's side after the South's military fired warning shots amid heightened tensions over Pyongyang's trash-carrying balloon campaign.

Military watchers speculate the incident could be related to the North's wall construction. At the time of the border incursion, the North Korean soldiers were carrying work tools, such as pickaxes and shovels.

The border crossing came amid heightened cross-border tensions set off by the North's recent trash-carrying balloon campaign.

The MDL horizontally bisects the DMZ, which has served as a buffer zone between the two Koreas since the 1950-53 Korean War ended in an armistice, not a peace treaty.