Fourteen crew members were killed and five are missing after two ships caught fire in the Kerch Strait near Crimea on Monday, authorities said, with a rescue operation still underway.
"There are 11 bodies," Alexei Kravchenko, spokesman for Russia's Federal Agency for Maritime and River Transport, told AFP.
"Another three people went under water before rescue workers' eyes."
Those three are most likely dead, he said.
The blaze on the two ships, unleashed by an explosion, occurred off the southern coast of Crimea, at the entrance to the Kerch Strait, which connects the Black Sea and the Sea of Azov.
The ships, Candy (Venice) and Maestro, both bore Tanzanian flags and one was a gas tanker. They were crewed by 31 people — 16 Turkish nationals and 15 Indian nationals.
Twelve people 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. Reports said some who jumped overboard suffered burn wounds and hypothermia.
"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 Kerch Strait between Russian-annexed Crimea and southern Russia controls access from the Black Sea to the Azov Sea, where there are both Russian and Ukrainian ports.
Ships must often wait weeks before being allowed to pass through the Kerch Strait to the Sea of Azov. Ukraine has alleged that ships that want to sail on to Ukrainian harbors are hassled by Russian authorities.
In November, Russia detained three Ukrainian navy vessels and their crews in the Black Sea near the Kerch Strait, fuelling tensions between the two countries. Russia annexed Crimea from Ukraine in 2014.
