Mayors representing cities along South Korea’s border with North Korea on Friday urged Seoul to take “strong measures” to stop and punish activists attempting to send anti-regime leaflets to the North.

The move follows a high-level statement from Pyongyang on Thursday threatening to withdraw from symbolic inter-Korean projects unless the “anti-DPRK” activities were stopped.

“We, all mayors of the border area mayor council, urge the government to devise strong measures that can eradicate acts of scattering flyers to North Korea,” the joint letter delivered to unification minister Kim Yeon-chul on Friday said.