The State Department says the United States has proposed the sale of a $3.5 billion Patriot missile system to Turkey.

Congress has been notified of the proposed sale, aimed at boosting Ankara's air- and missile-defense capabilities.

The State Department says the deal "will contribute to the foreign policy and national security of the United States by improving the security of a key NATO ally on the front lines of the fight against terrorism."

The proposal comes after NATO member Turkey announced it was buying S-400 surface-to-air missile systems from Russia.

Washington has complained that the Russian system is not compatible with weapons used by other NATO allies.

The State Department says, "The proposed sale will enhance Turkey's interoperability with the United States and NATO."

Asked about the American move, Kremlin spokesman Dmitry Peskov said on December 19 that Russia's contract to supply Ankara with S-400 missile systems is being fulfilled regardless of Turkey's decision about the Patriot missile system.

With reporting by Reuters, AP, and Interfax