Six ships were attacked in 10 days in the Black Sea by drones suspected of being Russian (among them the Able and Chelsea-3), an escalation in a maritime conflict that is pressing on the borders of the North Atlantic Treaty Organization (NATO), according to The Guardian.

By Thursday, the attacks had encroached on Romania’s airspace and territorial waters; the Turkish-owned Alfa Watan, bound for the Romanian port of Sulina, had been struck by suspected Russian air and naval drones. Two chemical tankers, Wine Trader and Yara J, were attacked and left burning, and seven more sailors were missing.