A cargo ship sank and its crew went missing after drones attacked two merchant vessels in Bulgaria's exclusive economic zone in the Black Sea on October 6, Bulgarian officials said, raising concerns about the widening risks to civilian shipping from the war in Ukraine.

The vessels were attacked by air and sea drones at about 3 a.m. local time, around 70 nautical miles (130 km) east of the Bulgarian coastal town of Byala, Prime Minister Rumen Radev said.

One of the ships, the Togo-flagged Alfa Watan, sank. The vessel was Turkish-owned and was reportedly heading towards the Romanian port of Sulina. Its cargo and the number of people aboard were not immediately clear.

A Bulgarian vessel that went to the scene found an overturned lifeboat, life jackets and rafts but no crew members, Radev said. A search-and-rescue operation was under way.

The second vessel, the grain carrier Able, was hit and caught fire. Its 18 crew members - 11 Turkish and seven Indian nationals - were rescued by the Bulgarian roll-on/roll-off vessel Dioscuria, Radev said. Two crew members, including the captain, were seriously injured and taken to a military hospital in Varna.

"This is an absolutely unacceptable attack, a gross violation of international law and maritime affairs," Radev said at an emergency government meeting, BTA reported.

He said it was not immediately known whether the drones had originated from Russia or Ukraine.

Rising Black Sea risks

The incident comes as attacks on commercial shipping in the Black Sea have increased, with vessels carrying Ukrainian grain among those hit, and as European governments warn of a growing threat from Russian hybrid warfare.

European Commission President Ursula von der Leyen condemned the attacks, saying the situation in the Black Sea was becoming increasingly concerning.

"Commercial ships – and innocent crews – are being targeted, with lives already lost at sea," von der Leyen said on X. "Attacks on civilian shipping are unacceptable."

She noted that the incident came a day after a Turkish ship carrying Ukrainian grain was hit and sunk off Romania.

Radev did not directly blame Russia for Tuesday's attack, but linked the incident to the war in Ukraine.

"As I have long maintained, if this war is not halted in time, it will continue to expand its geographical reach and increase risks, including the risk of a global conflict," he said, according to BNR.

Bulgaria, a NATO member, does not border Russia or Ukraine but occupies a strategically important position on the western Black Sea. Its exclusive economic zone covers more than 29,000 square kilometres.

President Iliana Iotova will convene Bulgaria's Consultative Council on National Security on October 7 to discuss risks to maritime security and critical infrastructure stemming from the war in Ukraine, her office said.

The Bulgarian authorities were also monitoring the sunken vessel and the surrounding area for possible environmental risks, Radev said.

The incident followed a separate attack near Ukraine's port of Odesa in which a Marshall Islands-flagged vessel was reportedly hit, killing one crew member and injuring seven others.