Istanbul: 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 3 a.m. (0000 GMT), the ​Istanbul governor's office said in a statement.Also read: 'Tanker for tanker': Trump's new retaliation policy behind US strikes on IranCoastguard 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 a.m. and the captain of the Alsu informed the rescue centre at 03:41 a.m. that it had collided with an unidentified vessel at around 03:15 a.m.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.