North Korea on Friday issued a strong warning against drone flights crossing from South Korea. It vowed a "terrible response" if such incursions continue. South Korean President Lee Jae Myung has tried to improve ties with the North since his election last year. He criticized the previous administration for sending drones to spread propaganda over Pyongyang. Despite this, North Korea said it shot down a South Korean surveillance drone last month. This incident threatens to spoil the peace efforts. Kim Yo-jong, powerful sister of North Korean leader Kim Jong-un, warned of harsh punishment if drones do not stop. "I give advance warning that reoccurrence of such provocation as violating the inalienable sovereignty of the DPRK will surely provoke a terrible response," Kim said in a statement on state news agency KCNA. She added, "We don’t care who the very manipulator of the drone infiltration into the airspace of the DPRK is and whether it is an individual or a civilian organisation." Her statement signals tough times ahead for inter-Korean relations if drone flights continue.