Fourteen people were killed and five were missing off Russia-annexed Crimea in the Black Sea on Monday after a fire engulfed gas tankers sailing under the flag of Tanzania, authorities said.

"There are 11 bodies," Alexei Kravchenko, spokesman for Russia’s Federal Agency for Maritime and River Transport, said.

"Another three people went under water before rescue workers’ eyes."

Those three are most likely dead, he said.

The ships, the Kandy and the Maestro, had crews of nationals from India and Turkey.

The Kandy had a 17-strong crew, while the Maestro carried 14 people.

Twelve people had survived, Kravchenko said, but "no one knows where the other five people are".

The fire broke out when one vessel was transferring fuel to the other, driving both crews to jump overboard, said Kravchenko.

"A search-and-rescue operation is continuing," Kravchenko added.

Authorities in the Crimean city of Kerch were preparing to receive the victims, he added.

The accident struck close to the Kerch Strait, which has become a new flashpoint in tensions between Russia and Ukraine.

In November, Russia fired on and seized three Ukrainian navy vessels in the Kerch Strait as they tried to pass from the Black Sea to the Azov Sea.