British fighter jets flew over Poland on Friday night as part of AN operation to bolster NATO’s eastern flank after Russian drones violated Polish airspace earlier this month, according to the UK Ministry of Defense.

NATO member Poland will not hesitate to shoot down objects that violate its airspace and pose a threat, but will take a more cautious approach to situations that are less clear-cut, the prime minister said on Monday.

Ukraine has increasingly taken aim at Russia’s refineries. Russia is the world’s second-largest oil exporter, with revenue from the sector crucial for its war effort. Sustained Ukrainian drone strikes as well as a seasonal rise in demand recently have brought shortages at the pumps.

After multiple Russian drones violated NATO airspace, Europe faces questions over the alliance’s long-term ability to deal with this growing threat.