Russia has diverted a warship to protect the Syrian coast and vowed to bolster Bashar al-Assad's missile defences against more US strikes, risking a confrontation between the former Cold World foes.

The Admiral Grigorevich, a cruise missile-carrying frigate, passed through the Bosporus en-route to Russia's Syrian navy base at Tartus on Friday.

The Grigorevich, which carries Moscow's state-of-the-art Kalibr cruise missiles, was taking part in joint exercises in the Black Sea with the Turkish navy when it was ordered to turn around.