SEOUL, South Korea (AP) — South Korea’s military says soldiers fired warning shots to repel North Korean soldiers who temporarily crossed the rivals’ land border for the second time this month. South Korea’s Joint Chiefs of Staff said around 20 to 30 North Korean soldiers briefly crossed the military demarcation line Tuesday morning. It said the North Korean soldiers retreated after the South broadcasted warnings and fired warning shots. The South’s military has observed increased North Korean construction activities along their heavily armed border to install suspected anti-tank barriers, reinforce roads and plant land mines. The South says the work has continued despite several explosions caused by mines that killed or injured an unspecified number of North Korean soldiers.