South Korea sent drones across the border into North Korea for the first time Monday, an unprecedented tit-for-tat military move after Kim Jong Un’s regime dispatched five unmanned aerial vehicles into its airspace.

The exchange of drones, which briefly stopped flights from taking off at major airports near Seoul, came as Kim opened a major political meeting to set security, economic and political policy for the coming year, the official Korean Central News Agency reported Tuesday. He has spent the past year improving his atomic arsenal, showing no interest in returning to nuclear disarmament talks that have been stalled for three years.

Kim’s regime sent five drones across the border on Monday, the first time he has done so in more than five years. The first one crossed the border at 10:25 a.m. and returned after flying for about three hours. Four more were detected Monday afternoon and later vanished from radar, South Korea’s Joint Chiefs of Staff said.