The Ukrainian-owned Chelsea 3, sailing under the Maltese flag, was struck by three drones at about 1:30 a.m. on Tuesday, approximately 18 nautical miles (33 kilometers) off Bulgaria’s coast, Radev said.

The vessel sustained damage, but all 15 crew members — seven Ukrainians and eight Azerbaijanis — were rescued.

Five crew members received medical assistance, although none was seriously injured, according to Radev, who also confirmed that one crew member from the vessel that sank was rescued.

Describing the incident as “absolutely unacceptable,” Radev warned that the attack had occurred in the immediate vicinity of Bulgarian territorial waters.

“Another attack in the immediate vicinity of Bulgarian territorial waters,” he said in a televised address, adding that he was due to speak with NATO Secretary-General Mark Rutte.

Radev said there was no clear evidence identifying those responsible for the attack.