North Korean soldiers have been injured and killed in landmine explosions on their side of the demilitarized zone that divides the Korean Peninsula, the South Korean military said Tuesday.

“The North Korean military is pushing ahead with the DMZ operations despite a series of fatal explosions of landmines,” an official at South Korea’s Joint Chiefs of Staff told reporters.

The JCS said its closed-circuit TV captured smoke near the sites where the landmines are believed to have exploded.

North Korea began restoring guard posts and other facilities along the border after announcing it would withdraw from the 2018 inter-Korean military pact to reduce frontline tensions last November. Earlier this year, North Korea also removed streetlights along the roads connecting to the South.

The South’s JCS believes North Korea is deploying a large number of its troops for its project to “turn the DMZ into a barren land” by planting landmines and reinforcing tactical roads.

The JCS thinks the front-line structures the North Korean military is building appear to be anti-tank barriers.

“The recent activities by the North Korean military seem to be measures to strengthen internal controls, such as preventing its people from traveling to and back from South Korea,” the JCS official explained.

“They are burying landmines mainly in areas where people have crossed the border in the past. The ‘barren land’ is being created to facilitate surveillance.”

Early Tuesday, North Korean soldiers also briefly crossed into South Korea before retreating after the South fired warning shots, the JCS said. This marks the second border violation by the North in less than two weeks.