A civilian cargo ship has sunk and its crew is missing after being attacked by drones in the Black Sea, Bulgarian officials say.

A second vessel - carrying grain - was also hit within Bulgaria's exclusive economic zone, Prime Minister Rumen Radev said. Its 18 crew were rescued.

Radev did not say who was behind the attacks, which come as Russia intensifies strikes in the Black Sea, particularly the eastern side, as part of its war against Ukraine.