NATO fighter jets were scrambled to intercept five Russian military aircraft flying over the Baltic Sea without flight plans or active transponders, the Latvian Air Force confirmed on Saturday.

The Russian planes were identified on two separate occasions, on Friday and Saturday, prompting a rapid response from NATO’s Baltic Air Policing mission.

According to the Latvian Air Force, the Russian jets were detected flying in international airspace near the Baltic states, but had not activated their transponders, an electronic system that helps maintain safe air traffic control.

“Russian jets regularly enter the airspace above the Baltics with transponders switched off, likely to test the response of NATO states,” The Kyiv Independent reported, citing past instances of similar activities by Russian aircraft.