The Russian navy created a de facto aerial defense zone to protect its Syria bases
The "Black Sea-based Russian surface action group" in the eastern Mediterranean are based along Syria's coast by the Bassel al-Assad Airport and the Tartous naval facility, which is home to a Russian port.
The ships are intended to provide aerial defense for the Russian aircraft that Moscow is currently using to carry out airstrikes against Syrian rebels throughout the country.
Although not directly stated by official channels, the aerial defense zone is likely intended as defense against hypothetical US or coalition airstrikes against the Russian airstrips.