Poland said on Friday that its jets intercepted a Russian reconnaissance plane over the Baltic Sea for the third time this week.

The aircraft did not violate Polish airspace, the Operational Command of the Polish Army said, but as in previous incidents it had no filed flight plan and its transponders were turned off.

“This is already the third such situation this week, confirming the increasing activity of Russian aviation in the Baltic region,” the army said.

Countries on Nato’s eastern flank have been on high alert for potential airspace incursions since September when three Russian military jets violated Estonia’s airspace for 12 minutes just days after more than 20 Russian drones had entered Polish airspace.