Illustration: Turkey with the Straits and Sea of Marmara area in red, by “The Emirr.” Dardanelles to West of the Sea of Marmara and Bosphorus to the East. 

Following from the German Navy blog, Marine Forum, 16 May,

“After Turkish intelligence learned of Islamic State plans for attacks on Russian warships from ashore, Turkish authorities have beefed up security along the Turkish Straits and augmented escorts by Turkish Coast Guard.”

A number of the Russian Navy transits carry supplies to the Syrian Government Forces and Russian forces operating in Syria.