One merchant ship has sunk and its crew is missing after a drone attack in the Black Sea, Bulgarian officials say. This was reported by Qazaqyia.kz citing BBC News.

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

Radev did not say who was behind the attacks, which come as Russia intensifies strikes in the Black Sea 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.

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.

A Bulgarian vessel was close by, but when they arrived all they saw was an overturned boat and rafts as well as life jackets. It was not clear how many crew members were on board. A search operation has been launched. The vessel's cargo was not immediately known.

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. The vessel was carrying grain.

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."

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.