South Korea said Monday it has detected signs that North Korea is preparing to destroy the northern parts of inter-Korean roads no longer in use, as the rivals are embroiled in soaring tensions over North Korea’s claim that South Korea flew drones over its territory.

South Korea’s military said Monday it has found North Korea is engaging in works to prepare for the explosions. It’s not clear how much parts of the roads North Korea would destroy.

The development comes as North Korea has accused South Korea of launching drones to drop propaganda leaflets over Pyongyang three times this month and threatened to respond with force if it happened again.

In a statement Sunday, the North’s Defense Ministry said that the military had issued a preliminary operation order to artillery and other army units near the border with South Korea to “get fully ready to open fire.” The spokesperson said that the entire South Korean territory “might turn into piles of ashes” following the North’s powerful attack.

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

Last week, North Korea said it will permanently block its border with South Korea and build front-line defense structures to cope with “confrontational hysteria” by South Korean and U.S. forces.

Read: North Korea vows to block border with South Korea, to build front-line defense structures

Tensions on the Korean Peninsula are at their highest point in years, with North Korea continuing a run of provocative weapons tests and South Korea and the U.S. expanding their military drills. KCNA said North Korea on Tuesday tested a long-range artillery system that observers say pose a direct threat to Seoul, the South Korean capital, which is only an hour’s drive from the border.