North Korea’s printing shops have been working overtime to revive a favourite weapon of Cold War-era psychological warfare: sending millions of propaganda leaflets across the world’s most heavily armed border and scattering them over South Korea.

The tit-for-tat move was announced on Monday by the North, which has become incensed by the leaflets that defectors from the North have sent from the South to their Communist home country in recent months, the North’s official news agency said.

The pledge by the North to retaliate presents another threat to the fragile détente on the Korean Peninsula.

North Korea said it was preparing 3,000 balloons to carry the leaflets across the border, along with cigarette butts and other trash. The defectors have continued their propaganda efforts despite protests from Pyongyang and inter-Korean agreements to stop them.

“The largest-ever distribution of leaflets against the enemy are almost complete,” the North’s official Korean Central News Agency reported, adding that publishing and printing houses in Pyongyang had printed 12 million leaflets, with those in provinces preparing to print millions more.

“The time for retaliatory punishment is drawing near,” it added. “South Korea has to face the music.”

North Korea has been expressing increasingly growing frustration with South Korea and the US, especially since the second summit meeting between the North’s leader, Kim Jong-un, and President Trump collapsed in Vietnam in February last year.

This month, it seized upon the leaflets sent by activists in South Korea to start reversing the détente, cutting off all communications lines with South Korea and blowing up a joint liaison office it had operated with the South in the past two years.

Leaflets have been a common psychological warfare tool used by both Koreas since the 1950-53 Korean War.