North Korea threatened on Thursday to end an inter-Korean military agreement reached in 2018 to reduce tensions if the South fails to prevent activists from flying anti-Pyongyang leaflets over the border.

Axar.az reports citing foreign media that Kim Yo Jong, the powerful sister of North Korea’s leader, also said the North could permanently shut a liaison office with the South and an inter-Korean factory park in the border town of Kaesong, which have been major symbols of reconciliation between the Koreas.

North Korean defectors and other activists in recent weeks have used balloons to fly leaflets criticizing the North’s authoritarian leader Kim Jong Un over his nuclear ambitions and dismal human rights record.