LONDON: Fourteen Syrians were among the crew of 16 aboard a cargo ship that sank off the coast of Bulgaria after a drone attack in the early hours of Tuesday, the Syrian Sailors’ Union said on Tuesday as the search for survivors continued.

The Guinea-Bissau-flagged HS Dignity sank 15 nautical miles east of the Bulgarian village of Krapets following one of two overnight attacks on merchant vessels in the Black Sea, Bulgarian officials said.

The sailors’ union told Alikhbariah TV in Syria that the ship was struck by an aerial drone and two unmanned surface vessels. Five crew members were rescued, and six boats and two helicopters continue to search for the remaining 11, it added.

In the other attack, three drones struck the Ukrainian-owned, Maltese-flagged Chelsea 3 in waters near the Bulgarian coastal town of Byala, authorities said. Bulgarian border police rescued all 15 crew members — seven Ukrainians and eight Azerbaijanis — and brought them to the port of city of Varna. Some were injured and received medical assistance, authorities said.

The attacks took place a week after aerial and maritime drones struck two other commercial vessels in Bulgaria’s exclusive economic maritime zone, about 70 nautical miles east of Byala, on Oct. 6. The Togo-flagged Alpha Watan sank and the Palau-flagged Ebel caught fire, the Bulgarian government said.

All 18 crew members aboard the Ebel were rescued. Accounts differ on how many people were aboard the Alpha Watan; Syrian authorities said nine Syrians and one Egyptian comprised the crew, while Bulgarian state television reported that 12 Syrian sailors were on board. Syrian authorities called on Oct. 8 for an “independent and transparent” investigation into the sinking.

The attacks last week also prompted Bulgarian authorities to summon the Russian ambassador. The Ukrainian foreign minister, Andrii Sybiha, accused Moscow of targeting commercial vessels and threatening maritime security in the Black Sea.

However, the Bulgarian prime minister, Rumen Radev, said his country lacked clear and conclusive evidence of who carried out the attacks, though he added that their consequences benefited Russia.

Moscow said it had no knowledge of the latest attacks. The Russian Embassy in Bulgaria previously denied any Russian responsibility for the last week’s strikes.