NATO fears "Moscow's eyes" on the territory of its member states, as the maintenance —400 anti-aircraft missile systems purchased by Turkey will require Russia's assistance, German Spiegel magazine wrote.
The military bloc has great concerns that Moscow might be able to access sensitive information, including data on the new American multifunctional fighter F-35.
Ankara is reportedly going to acquire more than a hundred aircraft of this type.
In its turn, Washington, has been trying in vain to persuade Turkey to buy the US-made MIM-104 Patriot surface-to-air-missiles, citing the S-400s' incompatibility with NATO standards.
Earlier, Turkish Undersecretary for Defense Industries (SMM) Ismail Demir told Turkey's Anadolu News Agency that Russia would supply its S-400 missile systems to Ankara within the agreed time frame.
Commenting on Washington's possible sanctions against Ankara over its planned purchase of the S-400 systems, Demir underscored that the Turkish government is aware of such reports and that the leaders of Turkey and Russia have made a clear decision which will not be affected by sanctions.
Moscow and Ankara inked a loan agreement for the acquisition of four S-400 air defense missile systems in December 2017. The initial delivery of the Russian-made advanced missile system to Turkey is planned for the first quarter of 2020.