South Korea's presidential Blue House on Thursday warned of a "thorough crackdown" against activists sending anti-Pyongyang leaflets across the border, after Human Rights Watch denounced it as "shameful" for seeking to block such activities.

The leaflets -- usually attached to hot air balloons or floated in bottles -- criticise North Korean leader Kim Jong Un over human rights abuses and his nuclear ambitions.

Since last week Pyongyang has issued a series of vitriolic denunciations of the South over the leaflets -- something defectors do on a regular basis -- and on Tuesday it announced it was severing all official communication links with the South.

Moon is a former human rights lawyer and earlier this week lauded South Korean democracy activists, but his office expressed "strong regret" over the leaflet launches, adding the government would mount a "thorough crackdown" against them.

The campaigns are illegal and do not help the "efforts to achieve peace and prosperity of the Korean peninsula", said Kim You-geun, a national security official at the Blue House.

Seoul's unification ministry filed a police complaint against two defector groups Thursday over leaflet launches.

Activists have regularly released balloons carrying anti-Pyongyang leaflets across the border to North Korea
Activists have regularly released balloons carrying anti-Pyongyang leaflets across the border to North Korea AFP / JUNG YEON-JE

Human Rights Watch condemned the ministry's decision, accusing Seoul of "kow-towing" to Pyongyang's threats.

"Instead of proposing a blanket ban on sending balloons with messages and materials to the North, President Moon should publicly demand that North Korea respect freedom of expression and stop censoring what North Koreans can see," said Phil Robertson, the group's deputy Asia director, in a statement.

"President Moon and his colleagues fought for much of their lives and careers to protect human rights in South Korea," he added.

"It is shameful how President Moon and his government are totally unwilling to stand up for the rights of North Koreans."

Seoul's moves come after Kim Yo Jong, the North Korean leader's powerful younger sister and key adviser, condemned the leaflet launches last week.

Pyongyang has since issued a series of statements and held several citizens' rallies on the subject, with the official KCNA news agency describing leaflet scattering as "an act of a preemptive attack that precedes a war".