A Greek container ship has sunk after being hit by naval drones off Bulgaria on Wednesday, the Bulgarian government said, in the latest of a series of strikes on ships in the western Black Sea this month.

It was not immediately clear who had carried out the attack, which added to growing concerns about the impact of Russia’s war in Ukraine on commercial shipping in the area.

The Liberian-flagged CONTSHIP JET, loaded with 421 containers, was badly damaged by three sea drones as it passed through Bulgaria’s Exclusive Economic Zone (EEZ) – but outside its territorial waters – en route from Haifa in Israel to Constanța in Romania.

The crew of 14 Filipinos and four Ukrainians were all rescued and transferred safely to the Bulgarian port of Varna, Contships Management said.

At least six ships have been attacked by drones off Bulgaria this month, and about two dozen seafarers are feared dead, according to government statements.

Bulgaria, a member of the European Union and NATO, has sought assistance to bolster security off its Black Sea coast.

Bulgarian naval commander Kiril Mihaylov told national television authorities had been alerted to a possible drone attack by a French patrol aircraft, but had been unable to prevent it.

The government said that initial inspections on Thursday had found no visible traces of pollution from the ship, whose cargo included lithium-ion batteries, corrosive liquids and acids and industrial components.