South Korean authorities have blocked activists from sending leaflet balloons across the border, following threats from North Korea to launch an attack if any balloons entered its airspace.

Security forces faced off on Monday with a group of North Korean defectors near Imjingak Park, just south the the Demilitarized Zone separating the North and South. The activists had planned to launch giant balloons carrying anti-Pyongyang leaflets into the North.

The North's official Korean Central News Agency said Friday that the Western Front Command of the Korean People's Army would conduct a “merciless strike” without warning if the leaflets were sent.

South Korea's military was put on high alert and the South's Yonhap news agency reported that extra artillery and rocket launchers were sent to the area to counter any attack.

The KCNA had accused South Korea's military of directly engineering propaganda, but Seoul has said it has nothing to do with the launches.

North Korea has lashed out at previous balloon launches, threatening to shell South Korea in response to such actions, which it calls deliberate provocation by Seoul that could lead to war.