North Korea’s military has been constructing roads and walls within the Demilitarized Zone (DMZ) that separates it from South Korea, according to a report by the Yonhap news agency on Saturday.

The construction is occurring north of the Military Demarcation Line (MDL), which runs through the center of the DMZ, as cited by an unnamed military source from the South Korean agency.

This report comes on the heels of an incident last week when South Korean forces fired warning shots after North Korean soldiers briefly crossed the MDL.

South Korean authorities suggested the incident was likely accidental, with Yonhap quoting a military spokesperson who noted that some of the North Koreans were carrying work tools.

“Recently, the North Korean military has been erecting walls, digging, and constructing roads in some areas between the Military Demarcation Line (MDL) and the Northern Limit Line within the DMZ,” the military source said, according to Yonhap on Saturday.

The purpose of the construction remains unclear, the source told Yonhap.

When questioned about the report, the South Korean military stated it is “closely tracking and monitoring the activities of the North Korean military” and that “further analysis is required.”

The South Korean military also mentioned it could not disclose its response to these actions “to ensure the safety of the personnel involved in the operation,” without providing additional details.