A Russian-flagged tanker and a Turkish fishing boat collided in the Black Sea near Istanbul early Friday, sinking the fishing vessel.
Istanbul governor's office said in a statement three Turkish citizens are missing while three were rescued by fisherman after the collision.
Authorities were searching for the missing with divers and boats.
The Russian ship Glard 2 was en route from Rostov-on-Don in Russia to Izmir in western Turkey, according to marine traffic trackers.
It is categorized as an oil and chemical tanker.
Heavy fog has prompted the temporary closure of Istanbul's Bosphorus strait, the governor's office said on Twitter.