One of the cargo ships that came under attack was the Ukrainian-operated coastal freighter Chelsea 3, registered under the flag of Malta. Bulgarian Prime Minister Rumen Radev told the Russian ambassador in Sofia that “it makes no sense for Ukraine to destroy its own ships in the Black Sea, and the results of these strikes favor Russia’s interests,” according to The Associated Press.

All 15 members of the Ukrainian-owned Chelsea 3 were safely evacuated by Bulgarian rescue operations after the ship was reportedly struck by three aerial drones.

But authorities were still searching for 11 missing crew members of the Guinea-Bissau-flagged cargo ship Dignity, which sank after coming under attack, The Associated Press reported. Five mariners were initially rescued from that ship.

Pattern of attacks and countermeasures

These incidents mark the latest in a growing series of unprecedented attacks on Black Sea shipping in the exclusive economic zones of NATO countries. One week earlier, aerial and sea drones sank the Togo-flagged cargo ship Alfa Watan and left the Palau-flagged cargo ship Able afire about 80 miles (128 kilometers) off the coast of Bulgaria on October 6.

The 18 crew members of the Able were rescued—two required hospitalization—but the 10-member crew of the Alfa Watan has been presumed dead after rescuers failed to find any survivors.

Such incidents first began when the Turkish-owned cargo ship Royad Mammadov came under drone attack and eventually sank near Romania’s Black Sea coast on October 5. All 11 crew members were rescued.

In response to the attacks, Bulgaria has stepped up cooperation with affected NATO member countries and the alliance as a whole. On October 14, Bulgarian Defense Minister Dimitar Stoyanov told the national parliament that the country would join NATO’s Drone Edge initiative that includes member countries committing a total of $40 billion toward developing more counter-drone capabilities over the next five years, according to independent news publication The Sofia Globe.

NATO officials began examining the possibility of moving existing anti-drone countermeasures to the region after receiving requests from Bulgaria, Romania, and Turkey, diplomats told Politico. The three countries are also cooperating on an initiative to safeguard freedom of navigation in the Black Sea, which Bulgaria Prime Minister Radev plans to present to the European Union this week.