South Korean authorities today blocked activists from flying anti-Pyongyang propaganda leaflets across the border after North Korea threatened to carry out a military strike on the balloon launch site.

Amid drizzling rain, vehicles carrying members of the Fighters for Free North Korea, a civic group composed of North Korean defectors, were blocked by police and soldiers from accessing Imjingak, near the Demilitarized Zone that divides the two Koreas, where they planned to launch helium balloons carrying some 200,000 leaflets at 11 am.

"We have placed an entry ban on civilians and vehicles taking into consideration safety concerns over North Korean warnings, clashes between anti- and pro- civic groups and complaints from residents," an official at the Gyeonggi Regional Police Agency was quoted as saying by Yonhap News Agency.

Yonhap reported that the activists protested against the entry ban.

On Friday, North Korea's military threatened that its army would launch a "merciless military strike," targeting the launch site if any move to send the leaflets across its border is detected, and it advised South Koreans residents living in the area to evacuate.

It said the scattering of leaflets "amounts to an undisguised psychological warfare, breach of the Korean Armistice Agreement and an unpardonable war provocation."

In response, South Korea has warned that its military is prepared to "completely destroy" the origin of a North Korean attack if it occurs.

Yonhap said the South Korean military has stepped up combat readiness by deploying artillery and tank brigades and combat air patrols by F-15K and KF-16 planes, according military officials.