The powerful sister of North Korean leader Kim Jong Un on Tuesday accused the U.S. of “illegally” sending spy planes into its exclusive economic zone (EEZ) eight times, delivering an implicit warning just a day after the country warned further flights could be shot down.

Kim Yo Jong, a senior official in her brother’s regime, warned that U.S. forces would face a "very critical flight" if they continue what the North claimed was an "illegal intrusion," the state-run Korean Central News Agency said in a dispatch, repeating an accusation Pyongyang made Monday that U.S. reconnaissance planes had violated its airspace by conducting surveillance flights.

She accused the U.S. military of flying a reconnaissance plane in the skies above the “economic water zone” 435 kilometers east of Tongchon in Gangwon Province and 276 km southeast of Uljin in North Gyeongsang Province, part of what she said was an “aerial espionage act.”