Drones sank two cargo ships and damaged a third in the exclusive economic zones of NATO member countries for the first time, killing at least two sailors. The attacks represent a notable escalation as Black Sea commercial shipping has increasingly come under attack during the Russian invasion of Ukraine, with Russia also being accused of carrying out “hybrid warfare” incidents across Europe.

The Togo-flagged cargo ship Alfa Watan sank, and the Palau-flagged cargo ship Able was set ablaze after being attacked by multiple sea and aerial drones about 80 miles (128 kilometers) off the coast of Bulgaria on October 6, according to the Associated Press. In an interview, Bulgarian Prime Minister Rumen Radev described the incidents as “an unacceptable attack in Bulgaria’s exclusive economic zone” and “a blatant violation of international law and maritime law.”

The fate of the Alfa Watan’s crew was not immediately known after the vessel sank. A Bulgarian ferry managed to evacuate the 18 crew members of the Able, with two severely injured mariners being flown to a hospital in the city of Varna.

This comes one day after drones struck the Turkish-owned cargo ship named Royad Mammadov within Romania’s exclusive economic zone in the Black Sea on October 5. That ship caught fire and eventually sank, with 11 crew members rescued by Romanian ships and helicopters while one crew member remains missing, according to Euronews. Two crew members were killed.

European Union president Ursula von der Leyen described the drone strikes on the three vessels in the Black Sea as “increasingly concerning” in a post on X. But she also cautioned that the circumstances of the attacks are “still being established” without naming a probable attacker.

“Attacks on civilian shipping are unacceptable,” Leyen wrote. “We stand in full solidarity with our Member States on the Eastern flank.”

By contrast, Ukrainian President Volodymyr Zelenskyy described a joint investigation with Bulgaria’s navy as finding that the latest strikes represented a “Russian combined attack using both sea and aerial drones,” Reuters reported. Zelenskyy also said that Ukraine’s military was not conducting operations in that part of the Black Sea.