South Korean Marines mark the first anniversary of North Korea's artillery attacks on Yeonpyeong Island on November 23.

Police  block South Korean activists near the Imjingak peace park, where they had planned to launch helium balloons carrying leaflets into North Korea, in the town of Paju on August 14, 2015

Police block South Korean activists near the Imjingak peace park, where they had planned to launch helium balloons carrying leaflets into North Korea, in the town of Paju on August 14, 2015 (AFP Photo/Jung Yeon-Je)

A statement, published jointly by front-line army units, said South Korea should not test the patience of North Korean soldiers.

It accused South Korean activists of having employed hit-and-run tactics in border areas to send anti-Pyongyang leaflets into the North more than a dozen times at night in June, July and August.

Such leaflet launches are “an open declaration of a war” against North Korea, the statement said.

About 100 South Korean police officers earlier formed human barriers to block vehicles carrying around 30 activists to the border town of Paju, from where they had planned to launch helium balloons carrying the leaflets into North Korea.

The pamphlets criticised and mocked the North’s ruling Kim dynasty and condemned a landmine attack blamed on Pyongyang that maimed two members of a South Korean army border patrol earlier this month.