Two vessels have been attacked by drones in the Black Sea near Bulgaria, with one ship sinking, Bulgarian Prime Minister Rumen Radev has said.

Eight Azerbaijani citizens were among the crew members aboard one of the vessels, according to the Bulgarian News Agency (BTA).

The Malta-flagged vessel Chelsea 3 was attacked by three aerial drones about 18 nautical miles east of the Bulgarian town of Byala.

“Another attack on merchant ships in the immediate vicinity of Bulgaria’s territorial waters is absolutely unacceptable,” Radev said.

All 15 crew members aboard the Chelsea 3 were rescued. The crew included seven Ukrainian citizens and eight Azerbaijanis. Five people sustained minor injuries.

The second vessel was attacked about 17 nautical miles east of the settlement of Krapets and subsequently sank, the Bulgarian prime minister said. One crew member was rescued.

Radev added that Bulgaria would seek international assistance to establish the origin of the drones. He stressed, however, that Sofia currently has no conclusive evidence to attribute responsibility for the attacks to any party.