Two Turkish-flagged commercial vessels collided off Istanbul in the Sea of Marmara on Wednesday and one of the ships, which had 10 crew on board, is believed to have sunk, the Istanbul governor's office and Turkish maritime affairs authority said.
The Alsu, en route to the Aegean port of Aliaga, and Tugberk Imamoglu, en route to the Black Sea port of Karadenizeregli, collided off the Silivri district of Istanbul at around 03:00 (00:00 GMT), the Istanbul governor's office said in a statement.
Coastguard ships and helicopters were sent to the scene for search and rescue, and there was no visible damage on the Alsu, the statement said. Authorities were unable to establish contact with the Tugberk Imamoglu and its 10 crew members.
One helicopter and 14 coastguard ships were conducting the search and rescue operation, the Transport Ministry's maritime affairs authority said.
It said its rescue centre received an emergency signal from the Tugberk Imamoglu at 03:26 and the captain of the Alsu informed the rescue centre at 03:41 that it had collided with an unidentified vessel at around 03:15.
The Tugberk Imamoglu was carrying some 4,200 tons of coiled steel, and the Alsu was carrying some 3,000 tons of engine oil when they collided in a busy ship traffic lane, the maritime affairs authority said.
It said that since the Tugberk Imamoglu had failed to respond to any calls and the crew could not be reached by their mobile phones, the authority assessed that it had sunk.
According to data from the Istanbul Chamber of Maritime Commerce, the Tugberk Imamoglu, owned by Zeybe Denizcilik, operates as a bulk carrier, while the Alsu, owned by Ozpulathane Deniz Tasimaciligi, operates as a chemical tanker.
(Reporting by Huseyin Hayatsever and Can Sezer; Editing by Christian Schmollinger, Daren Butler and Timothy Heritage)