"I would question just what the purpose of the system is," Cook said, The Washington Post reported.

He noted that Russia's declared goal in Syria is to fight extremism, but that the Islamic State group and the al-Qaida branch there don't have planes that could be targeted by such a system.

Read alsoRussia deploys advanced anti-missile system to Syria for first time – Fox NewsAsked if the system posed a threat to U.S. aircraft or crews, Cook said, "It depends on how the Russians plan to use it."

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He said the United States would continue to ensure the safety of its forces, which have the right to act in self-defense.