Bulgaria on Tuesday insisted that Russian aircraft had violated international rules while flying over the Black Sea.

The Bulgarian Ministry of Defense said Russian aircraft turned off their transponders — communication devices that allow an airplane to be located — and flew without previously announced flight schedules.

Bulgaria, a NATO member, was reacting to a statement on Monday by a spokesman for Russia's Defense Ministry who denied any such violations. He said Russian aircraft followed international rules and always kept their transponders on.

The exchange of accusations started on Sunday, when Bulgarian Defense Minister Nikolay Nenchev said in an interview with private Nova TV that there had been a rise in violations of Bulgarian airspace by Russian military and commercial aircraft.