The moment any such activity is detected, a "merciless military strike by the Western Front will be put into practice without warning," the North Korean Central News Agency (KCNA) quoted the Western Front Command of the North Korean People's Army as saying.

North Korea often threatens to fire at South Korean border facilities to stop "psychological warfare" attacks on its territory.

Defectors from the North who live in South Korea plan to send in leaflets on October 22. They often send balloons across the border with leaflets or even money as part of their campaign against what they call the dictatorship in the isolated socialist state.

South Korea vows it will not leave the threat unanswered, saying it would return fire.

"If (a North Korean strike) were to happen, there would be a perfect response against the source of the attack," Defense Minister Kim Kwan-jin told a parliamentary committee, Reuters reported.

KCNA also warned inhabitants of the border region to leave the area of Imjingak "in anticipation of possible damage."

Imjingak, a tourist pavilion located just south of the inter-Korean border village of Panmunjom, has become a popular site for South Korean activists to launch balloons carrying anti-Communist leaflets into North Korean territory.