South Korea called on activists Friday to immediately stop their campaign to send anti-Pyongyang propaganda leaflets across the border, vowing to take stern actions against it.

North Korean defector-activists' move to fly such leaflets is an "outright violation" of the spirit of a joint declaration adopted at last week's inter-Korean summit, according to Seoul's unification ministry.

"It is desirable that civic groups immediately suspend the campaign, to bring peace to the Korean Peninsula and ease military tensions, as the move heightens tensions along the border and serves as a cause for conflicts," the ministry said.

It said that the government will take stern measures to deter the move while strengthening communication with civic groups and cooperating with police.

North Korea is sensitive to North Korean defector-activists'

sending of leaflets via balloons due to concerns that ordinary North Koreans can get information about the repressive regime. The leaflet campaign is one of the main sources of tension between the two Koreas.

A group of North Korean defectors warned Friday that it will fly leaflets into North Korea on Saturday at noon in Paju, near the border.

The joint summit declaration said that the two Koreas agreed to stop all hostile acts along the border, including the leaflet campaign, starting May 1 to soothe military tensions.

The previous conservative administration said that Seoul cannot curb the leaflet launches as they are a matter of freedom of speech, but added the move should not pose serious threats to residents living near the border.

The liberal government under President Moon Jae-in previously said that the government is reviewing policy for the anti-North Korea leaflet campaign.

In October 2014, the North fired machine guns at balloons launched by activists. Some bullets landed in South Korean territory, but no one was hurt.

The two Koreas agreed in June 2004 to end propaganda campaigns, including border broadcasts and the sending of leaflets, under the liberal government of late former President Roh Moo-hyun. But civic activists kept sending balloons carrying leaflets, leading to backlash from North Korea. (Yonhap)