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BARBARA STARR, CNN PENTAGON CORRESPONDENT (voice-over): This is what worries U.S. pilots right now, the Russian S-400 anti-air missile system
being unloaded in Syria.
Russian President Vladimir Putin says this massive system will protect his pilots and aircraft in the wake of Turkey shooting down a Russian jet.
VLADIMIR PUTIN, RUSSIAN PRESIDENT (through translator): We need to have security for our air force, and that is why we have set up the modern
system, S-400. This is one of the most effective and efficient systems in the world.