Two ships were struck by drones in the Black Sea off Bulgaria on Tuesday, sinking one vessel and triggering emergency rescue operations, Bulgarian Prime Minister Rumen Radev said.

Rescuers evacuated all 15 crew members from one of the ships, the Malta-flagged Chelsea 3, with five receiving treatment for minor injuries, Radev said in a video posted on social media.

The crew comprised seven Ukrainian and eight Azerbaijani nationals. They reported that three aerial drones hit the ship’s deck and hull, he said.

One crew member from the sunken vessel was rescued, while Radev provided no details about the fate of the others aboard. The two attacks took place 18 miles (29 kilometres) and 17 miles (27.5 kilometres) from the Bulgarian coast.

Speaking at an emergency meeting of Bulgaria’s Security Council on Tuesday, Radev described the attacks as “absolutely unacceptable” and said his country was seeking assistance from NATO and the European Union.

Although authorities had no clear evidence identifying those responsible, Radev said that “logic, as I have already stated, indicates that the result is in favor of the Russian Federation.”

The latest attacks came exactly a week after sea and aerial drones struck two other commercial vessels in Bulgaria’s exclusive economic zone in the Black Sea. A Togo-flagged ship sank, while a Bulgarian ferry evacuated all 18 crew members from a Pulau-flagged vessel.

Ukrainian President Volodymyr Zelenskyy said Ukraine’s military had not conducted any operations near the sites of last week’s attacks and blamed Russia.

A day earlier, a Turkish-owned cargo ship carrying corn caught fire after a drone attack and later sank in the Black Sea off Romania’s coast. Two people were killed and 11 crew members were rescued, Romanian authorities said.

Russia did not immediately comment on Tuesday’s incident.

Radev said he would summon Russian Ambassador to Bulgaria Eleonora Mitrofanova for a meeting on Tuesday.