The North Korean Army on Tuesday warned that it will re-enter border areas demilitarised under a peace pact with South Korea, if defector groups there continue to send propaganda leaflets across the border, BBC reported. The so-called demilitarised zone is a buffer along the border between North and South Korea that has separated the two countries since the Korean War in the 1950s.

Leaflets critical of North Korean leader Kim Jong-un and the regime’s disregard for human rights have become a source of tension between the two Koreas recently. Last week, Pyongyang cut all communications with the South, including a hotline between the two nations’ leaders, after it was angered by the pamphlets sent by defector groups. The sending of leaflets across the border was also banned under the 2018 inter-Korean agreement.

The General Staff of the Korean People’s Army said it has been reviewing an “action plan” to advance into unspecified zones that had been demilitarised under a 2018 inter-Korean pact and “turn the front line into a fortress”.

“Our army will rapidly and thoroughly implement any decisions and orders of the Party and government,” the army said in a statement, according to Al Jazeera. The KPA also said North Korea would send its own propaganda into the South as part of a “large-scale leaflet scattering struggle against the enemy”.

South Korea’s defence ministry on Tuesday responded to the threats and said it was working with the United States to closely monitor military moves in the North.

The propaganda material is usually sent via balloons. Defector-led groups often send these leaflets along with food, one-dollar bills, mini radios and USB sticks containing South Korean drama and news. South Korea said the defector groups’ actions pose risks to residents living near the border and cause environmental damage.

Meanwhile, Kim Yo-jong, the sister of North Korean leader Kim Jong-un, said the North would demolish “a useless inter-Korean liaison office” in the border town of Kaesong and that she would leave it to the military to come up with the next step of retaliation. She promised to take action and said that “rubbish must be thrown into the dustbin”.

However, South Korea’s President Moon Jae-in urged Pyongyang on Monday to keep peace agreements reached by the two leaders and solve all issues via dialogue.