At least 10 people have died in a fire involving two Tanzania-flagged cargo vessels in the Black Sea near the Kerch Strait, authorities say.
Russian rescuers are trying to reach sailors who jumped overboard. Fourteen people have so far been rescued.
One ship is a gas tanker, and the fire reportedly followed an explosion, which set the other vessel on fire.
They were named as the Kandy, with a crew of 17 from Turkey and India, and Maestro, with 14 sailors.
The fire broke out when one vessel was transferring fuel to another, Russian maritime agency spokesman Alexei Kravchenko said, adding that this then forced several crew members to jump overboard.
The agency earlier said that "no signal from either one of the two captains" had been received.
The crew members were sailing in "neutral waters" in the Black Sea when the incident occurred, authorities said.
The Kerch Strait is a focus of tension between Russia and Ukraine.
In November, Russian border guards seized three Ukrainian naval vessels near the narrow channel, which links the Black Sea with the Sea of Azov.
A court in Russia has extended by three months the detention of 24 Ukrainian sailors captured in the incident. They are accused of illegally crossing into Russian territory.
Ukraine condemned the Russian move, denying that its ships had violated the navigation laws in the area. The strait lies off Ukraine's Crimea peninsula, which Russia annexed in 2014.