North Korea has accused U.S. spy planes of violating its air space and has threatened to shoot them down, ramping up tensions just before NATO leaders meet this week in Lithuania for their annual summit.

A spokesman for North Korea’s Defense Ministry said the U.S. was engaging in "the most undisguised nuclear blackmail” by planning to bring a nuclear-armed submarine to the Korean Peninsula and conducting "hostile espionage activities” by flying spy planes off its east and west coasts, the state’s Korean Central News Agency reported Monday.

North Korea claimed that drones and spy planes flew for eight straight days along its coasts, with aircraft violating its airspace. "There is no guarantee that such shocking accident as downing of the U.S. Air Force strategic reconnaissance plane will not happen,” it quoted the spokesman as saying.