North Korea’s military is building roads and walls inside the demilitarized zone, which separates it from the South, Yonhap news agency reported on Saturday.

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

The report comes after last week’s incident, when South Korean forces fired warning shots after North Korean troops briefly crossed the MDL.

South Korean officials said it was probably accidental, and Yonhap quoted a military spokesman as saying some of the North Koreans were carrying work tools.

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

The source told Yonhap that it was unclear what they were making.

Asked about the report, the South Korean military said in a statement that it was “closely monitoring the activities of the North Korean military” and that “further analysis is needed.”

It said it could not share South Korea’s response to the actions “to ensure the safety of operational personnel,” but did not provide further details.

South Korea’s spy agency told AFP this week that it had detected signs that North Korea was demolishing parts of a railway line linking the two countries.

After this the propaganda war between the two Koreas increased.

North Korea sent more than a thousand balloons carrying garbage into South Korea, saying they were sent in retaliation for propaganda balloons sent to the other side by anti-Pyongyang activists.

Following this, South Korea resumed streaming K-pop songs and news broadcasts into North Korea using loudspeakers installed along the border.

Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, threatened an unspecified “new counterattack” as the loudspeaker campaign resumed.

North Korea tightly controls the flow of information within its borders and is extremely sensitive about its people’s access to South Korean content, especially pop culture.

It has previously threatened to attack South Korean loudspeakers with artillery — a psychological warfare tactic that dates back to the 1950-53 Korean War.