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A Russian military spokesman said two Russian frigates - the Admiral Makarov and the Admiral Grigorovich - had left the port of Sevastopol on the peninsula of Crimea, which Russia annexed from Ukraine in 2014, and were transiting the Bosphorus and then the Dardanelles straits.

"The frigates Admiral Makarov and Admiral Grigorovich - armed with highly accurate Kalibr-NK missile systems - are carrying out a planned transit from Sevastopol," fleet spokesman Aleksei Rulev told the RIA Novosti and Interfax news agencies.

He did not specify where the ships were eventually heading, but Russian naval ships that pass through the Bosphorus normally head into the Mediterranean to back Moscow's operations in Syria.

The transit came hours after Ankara announced at least 33 Turkish soldiers were killed in Syria's Idlib province after a strike it blamed on Russia's ally Damascus. 

Russia said the troops were "where they should not have been" and had been among "terrorists". 

The incident has raised new questions about the sustainability of the alliance between Turkey and Russia to bring peace to Syria.

As a Black Sea littoral state, Russia is allowed to have its military ships pass through the Bosphorus under the 1936 Montreux Convention on the Straits.

Under its terms, Turkey can only block Russian naval shipping if war is declared or if it feels under an imminent threat of war.

The intense traffic of Russian vessels to and from Syria - known as the Syrian Express - has caused immense curiosity in Istanbul where the warships pass through the heart of the city in full view of ship-spotters.

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