Since leader Kim Jong Un declared the South his country’s “principal enemy” earlier this year, the North has laid fresh mines, erected anti-tank barriers, and deployed missiles capable of carrying nuclear warheads along the already heavily fortified border.

Last week, North Korea accused Seoul of using drones to drop anti-regime propaganda leaflets on the capital Pyongyang, with Kim convening a security meeting to direct a plan of “immediate military action” in response, state media reported Tuesday.

“North Korean has detonated parts of the Gyeongui and Donghae roads north of the Military Demarcation Line,” the Joint Chiefs of Staff said Tuesday, referring to inter-Korean infrastructure that once connected the two countries. The roads and railways have long been shuttered, but destroying them sends a clear message that Kim is not prepared to negotiate with the South, experts said.

Relations between the two Koreas are at their lowest point in years, with the North’s army saying last week it would permanently shut the southern border by “completely cutting off roads and railways” connected to the South and building “strong defence structures”.