NATO fighter jets took off after a Russian Mi-8 helicopter crossed into Polish airspace from the Kaliningrad exclave on Wednesday morning. Poland’s Operational Command of the Armed Forces said the transport helicopter remained inside for 42 seconds at 300 meters near Braniewo in the Warmia region by the Vistula Lagoon, and ground troops were placed on alert.

Warsaw said it believes the crossing tests the readiness of Polish air defenses.

Troops also searched near Malbork castle for remains of an object that air defense radars had detected, which could have been a balloon, a weather phenomenon, or a flock of birds.