While fully in place but not yet activated, the system, which was shipped in over the weekend, is a newer, modified version of the S-300VM, also known as the SA-23.
It expands Russia's anti-air capability in northwest Syria significantly. Though the US does not believe the Russians plan to target US pilots, one official called the development "a concern."
The system is located in a mountainous area that the US believes gives the Russians significant extended air coverage. The radar extends into Turkey and eastern Syria when it's placed on a mountain.
Fox News first reported the missile system being brought into Syria.
For months now, the Russians have had air defense that lets them see north to the Turkish border, so this expands the envelope potentially in a militarily significant manner.
US officials feel that Russians likely want to extend their air defense envelope out to Dier Ezzor to demonstrate to the world they and the Syrian regime are in control of wide swath of Syria.
But a US official with direct knowledge of the situation and the thinking inside the Administration says the anti-regime forces and other militia forces -- the majority of which form the Free Syrian Army, a main anti regime force -- are now estimated to total as much as 100,000 fighters across Syria.
This means the Syrian military would not likely be able to wipe them out and assert full control on the ground. The battlefield stalemate appears to have no solution at this point. The Administration is not considering military options against the Syrian regime, several officials say.
But there are growing signs that inside the Administration there is a rift. Several senior Pentagon officials are making it clear they do not favor military action. The State Department and the intelligence community are more open about the potential for some type of move to pressure Russia though its not clear what that would be.
On Friday, CIA Director John Brennan told Reuters "I think that pushing back against a bully is appropriate," adding, "I think that is very different than rushing in and bombing the hell out of a place."