Poland’s defense chief traveled to Kyiv on Thursday in search of Ukrainian help in developing his country's drone warfare capabilities, just over a week after Russian drones entered Polish airspace and exposed NATO’s vulnerability to drone warfare.

Estonian airspace was violated by three Russian jets on Friday, the country’s foreign ministry said, adding that it had summoned Russia’s chargé d’affaires over the incident.

Estonian foreign minister, Margus Tsahkna, said Russia had already entered Estonian airspace four times this year.