Drone attacks struck two cargo ships off Bulgaria’s Black Sea coast Tuesday, sinking one and leaving 11 sailors missing, Bulgarian officials said. Five people were rescued from the sunken vessel, while all 15 crew members aboard the second ship were evacuated.

The attacks prompted Bulgaria to seek help from NATO and European Union partners to investigate the strikes and strengthen surveillance of its surrounding waters and airspace. They follow attacks on two other commercial vessels in Bulgaria’s exclusive economic zone a week earlier.

The ship that sank was identified as the Guinea-Bissau-flagged HS Dignity, which carried a crew of 16. Bulgaria’s transport ministry said it was hit by a drone at about 1:20 a.m. Tuesday. Nearby merchant vessels helped rescue survivors.

The second vessel, the Malta-flagged Chelsea-3, was attacked off Byala. Its crew was brought ashore, with five people requiring medical attention.

Prime Minister Rumen Radev said Bulgaria lacked conclusive evidence establishing the drones’ origin, but argued that the attacks’ outcome benefited Russia. He raised the issue in a meeting with Russian Ambassador Eleonora Mitrofanova.

Russia’s ambassador said she had no knowledge of the attacks and suggested they could be provocations by third parties, Reuters reported. Bulgaria has not formally attributed responsibility.

Radev also spoke with European Commission President Ursula von der Leyen and NATO Secretary-General Mark Rutte. Bulgaria’s government said both expressed solidarity and readiness to help establish the circumstances of the attacks. Sofia is also seeking closer coordination with Romania and Turkey to improve maritime security.