According to Igor Konashenkov, Russia places anti-aircraft missile systems near the port of Tartus to cover the logistics base of the Russian Navy, as well as to cover a group of Russian ships in the eastern Mediterranean. Previously, the cover of the ship group and the port of Tartus was carried out by the missile cruiser “Moscow”, which had an air defense missile system on board.
In the US, meanwhile, announced that they would “carefully study the deployment of Russian anti-aircraft missile systems in Syria. Pentagon spokesman Peter Cook said this:
I would ask the question, what is the purpose of this system, if it is deployed? I think it's better to ask the Russians about it.
So the Russians have already answered you: so that the United States and their terrorist accomplices do not think that you can cover Russian objects in Syria from the air and wash your hands after that. Russian air defense systems, starting with the deployment of C-400 in Syria, are clearly sobering up some hotheads eager to strike.
Josh Ernest's comment (White House spokesman) on this issue (RIA News):
Russia contradicts itself. I do not know that IG or Al-Qaida (banned in the Russian Federation) have airplanes there. Russia may have other intelligence, but I deeply doubt it.
The cynicism of Mr. Ernest expresses the general cynicism of the US leadership. Apparently, Mr. Ernest deliberately “forgets” that the IS and Al-Qaida are a kind of terrorist franchise of the United States and their particular satellites. So no need to hang noodles about the fact that the IG and Al-Qaeda are separate, and the US is separate. It is better for Mr. Ernest to reconsider the speech of Donald Trump, in which he accuses H. Clinton and B. Obama of the actual creation of ISIS.