North Korea Friday threatened to respond with force against South Korean activists sending anti-Pyongyang leaflets and materials via balloon across the inter-Korean border.

Despite the Seoul government's request for restraint, anti-North Korea activist Park Sang-hak and his group on Thursday attempted to launch balloons carrying anti-Pyongyang leaflets and DVDs of the controversial U.S. movie "The Interview," a U.S. fictional comedy about a plot to assassinate North Korean leader Kim Jong-un. His attempt was scuttled by police.

Another activist group resumed sending anti-Pyongyang leaflets and the DVDs of the U.S. film to the North last week, a practice that is one of the main sources of tension between the two Koreas.

North Korea's propaganda website Uriminzokkiri criticized Seoul activists for launching and trying to send such balloons to the North, saying that the moves are tantamount to a declaration of war.

"We will bombard with blows of fire to (South Korea) at our disposal as they are making provocations," it said. "Our patience is wearing thin."

It said that the North's military is zeroing in on such activists with tension at its peak.

The North's threat came as Park revoked his pledge made in late March to suspend the leaflet campaign amid growing North Korean military threats and worsening public opinion against his plans in the South.

Seoul's unification ministry said that the government cannot curb the leaflet launch as it is a basic right of expressing the freedom of speech, but the move should not pose serious threats to residents living near the border.

"The Seoul government cannot forcefully deter the leaflet launch without legal grounds as the leaflet launch is a matter of freedom of speech," Lim Byeong-cheol, spokesman at the unification ministry, said at a press briefing on Friday.

"But the freedom of speech should also not put the safety of others at risk or disrupt public order. So the government will deal with this issue by taking into account both guaranteeing basic rights and protecting the safety of residents in a balanced manner."

In October last year, the North fired machine guns at balloons launched by activists. Some bullet rounds landed in the South's territory, but no one was hurt.

Activists in the South often send anti-Pyongyang propaganda leaflets, including U.S. dollar bills, using balloons across the border to criticize the North Korean regime. (Yonhap)