It was a rhetoric the North used for the first time after it did so 16 years ago.
"The South's carrying out psychological warfare amounts to the declaration of war. Our military response wouldn't be one to one-to-one response, but will be a merciless military attack that will even anticipate making Seoul a sea of fire," the North's General Staff of the Korean People's Army said, the Hankook Ilbo newspaper said.
North Korea last time used the rare rhetoric in 1994 during an inter-Korean negotiation.
In response, South Korea designated North Korea as its "main enemy" the following year.
The South Korean military so far has installed propaganda speakers at 11 locations along the Demilitarized Zone and also at checkpoints in the often disputed West Sea.
Source: Korea Times