US surprised by Russia's decision to send S-300 to Syria

On Tuesday, Russia sent an air-defense missile system S-300 to Syria. The deployment immediately raised questions from the Pentagon.

The US military will carefully track installation of S-300 air defense systems by Russia in Syria, the US Department of Defense spokesperson Peter Cook said.

"We are certainly aware of their announcement that they plan to install a system in Syria, and obviously we will carefully tack military developments and installations in Syria, particularly as they affect the coalition operations," Cook stated in a briefing. 

Peter Cook called on Russia to "explain" why it would deploy S-300 surface-to-air missile systems to Syria. 

"ISIL doesn't have an air force and neither does Nusra [Front], which are the two groups the Russians have said they're most concerned about," Cook said.