North Korean land mines could be swept into South Korea by flooding, South Korea’s military warned Wednesday, after the North has recently placed tens of thousands of additional deadly explosives along the rivals’ heavily fortified border.

North Korea’s minelaying is part of construction at the border that’s been going on since April, which also includes adding anti-tank barriers and reinforcing roads. South Korea officials believe North Korea aims to boost its frontline security posture and prevent its soldiers and citizens from defecting to South Korea.