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.

Moscow has not commented on the latest incident, a day after it was accused by Ukrainian President Volodymyr Zelensky of striking a civilian vessel carrying corn. EU Commission President Ursula von der Leyen said the attacks were "unacceptable".

Air and sea drones were involved in the attacks at about 03:00 (23:00 GMT), some 70 miles (112 km) off the coastal town of Byala in eastern Bulgaria, Radev said.

The sunken ship was flying under the flag of Togo, he said. Named the Alfa Watan, it was a Turkish-owned general cargo ship and was reportedly heading for Sulina, Romania.

It is not clear what the Alfa Watan was carrying or how many people were on board.

A Bulgarian vessel was close by, but when they arrived all they saw was an overturned boat and rafts as well as life jackets. A search operation has been launched.

Radev said Bulgarian sailors had rescued the crew members from the second ship - including two who were injured in a fire on board. Eleven were Turkish and seven Indian nationals. That ship, the Able, was flying the flag of Palau and transporting grain.

Its last reported position was five nautical miles (9km) from the Alfa Watan.

Radev condemned the strikes as "a flagrant violation of maritime law".

He told reporters: "Such an attack within the Bulgarian exclusive economic zone is absolutely unacceptable."

He did not accuse Russia of carrying out the strikes, but referred to the war in Ukraine, saying: "As I have long maintained, if this war is not halted in time, it will continue to expand its geographical reach and increase risks, including the risk of a global conflict."

European Commission President Ursula von der Leyen wrote on X: "The situation in the Black Sea is increasingly concerning.

"Commercial ships – and innocent crews – are being targeted, with lives already lost at sea," she said. "While exact circumstances of these incidents are still being established, the developments are very concerning."

"Attacks on civilian shipping are unacceptable."

Bulgaria's exclusive economic zone in the Black Sea is just over 29,000 sq km (11,197 sq miles), about 6% of the total size of the Black Sea.

A separate attack on Tuesday, near Odesa, saw a crew member killed and seven more injured. The ship was flying the flag of the Marshall Islands, according to Ukrainian media.

On Sunday, Russia's defence ministry said it would intensify its strikes across Ukraine in response to comments made by Zelensky earlier last week, in which he said Kyiv would ramp up its attacks on Russian oil refineries.

Russia launched a full-scale invasion of Ukraine in February 2022.