From Russia Today:

In November 2015, Moscow deployed its newest S-400 air defense missile system to Khmeimim in Syria as part of a security boost following the downing of a Russian jet by Turkey near the border with that country. At the time, Russian President Vladimir Putin said that the S-400 systems are not targeting Russia’s partners, “with whom we fight terrorists in Syria together.”

The S-400 is the most advanced anti-aircraft defense system in Russia.