The Batuhan A, which was bringing dry cargo from Marmara Island to Bursa in northwest Turkey, took on water and sank off Imrali island, south of Istanbul, officials said.

The Turkish navy detected the ship at a depth of 51 meters with a sonar device, the defence ministry said.

Bursa governor Mahmut Demirtas said authorities received an "emergency" signal from the vessel but then lost contact.

The transport ministry said an empty lifeboat was found during the search.

"The rescue operation continues to seek six crew members who are believed to be Turkish citizens," Demirtas said.

"The waves are reaching three meters. I think we will receive positive news as soon as possible in our search efforts," he added.

Helicopters in the region could not take off because of bad weather early in the morning.