North Korea is preparing to blow up roads crossing the heavily militarized border with South Korea, Seoul said.

troops out North Korea were seen working on the streets, the south's military spokesman said, and were already preparing for a detonation on Monday.

“They have installed screens on the road and are working behind those screens, preparing to blow up the roads,” South Korea's General Staff spokesman Lee Sung Joon said at a briefing.

The destruction of the roads would be in line with Kim Jong Un's push to sever ties with South Korea and officially make the country the North's main enemy.

It comes after the North accused this south to fire drones over its capital three times this month to drop propaganda leaflets.

The country's military warned of a “terrible catastrophe” if the drones were detected again and said it had placed eight armed artillery units at the border “on standby to open fire.”

Experts say the North is highly unlikely to launch large-scale pre-emptive attacks because its military is superior to the combined forces of the US and South Korea.

South Korea has refused to confirm whether it sent drones but warned it would punish North Korea if the safety of its citizens was threatened.

Officials in the south claim the north has erected anti-tank barriers, laid mines and reinforced roads on its side of the border since the start of the year, likely in an attempt to increase security at the front and prevent soldiers and citizens from defecting.

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Tensions between the two are at their highest level in years with North Korea continuing a series of weapons tests and South Korea expanding its military exercises alongside the US.

In January, Kim Jong Un ordered a revision of North Korea's constitution to remove the goal of peaceful unification of Korea and officially declare South Korea the country's “immutable main enemy.”

This was seen as a break with his predecessor's long-term goal of achieving a unified Korea on the North's terms.