North Korean soldiers crossed the military demarcation line and infiltrated into the South Korean side of the demilitarized zone (DMZ) at least five times so far this year.

The South Korean military said Tuesday that North Korean troops have stepped up infiltration, ambush and assault drills near the DMZ this year, taking advantage of the area’s thick woods.

The South Korean military said the South has beefed up surveillance using thermal observation and other equipment.

The military has also confirmed that three North Korean soldiers came down 600 meters south of the demarcation line last month and damaged a sign with guidelines on how North Korean soldiers can defect to the South.