North Korea's military has been building roads and walls inside the Demilitarized Zone that separates it from the South, the Yonhap news agency reported Saturday.

The construction activities are taking place north of the Military Demarcation Line (MDL) that runs through the middle of the DMZ, the South Korean agency said, citing an unnamed military source.

The report follows an incident last week when South Korean forces fired warning shots after North Korean soldiers briefly crossed the MDL.