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Ankara has gone ahead with its purchase of the Russian defence system despite threats of US sanctions.
Ankara received its first supply of S-400 missiles in July, despite a warning by the United States [aljazeera.com] about possible sanctions. The acquisition of the highly-advanced air defence system has led to a standoff between Turkey and its NATO [aljazeera.com] allies, especially the US.
Deliveries of the system are set to continue until April 2020 [aljazeera.com].
The modular S-400 is seen as one of the most advanced missile systems in the world, capable of tracking several targets simultaneously and ready to be fired within minutes.
The US has repeatedly said that the Russian system is incompatible with NATO systems and is a threat to the hi-tech F-35 fighter jets, which Turkey is also planning to buy.
Washington has said Turkey will not be allowed [aljazeera.com] to participate in the F-35 programme because of the Turkey-Russia deal.
The US has strongly urged Turkey to pull back from the deal - the first such move between a NATO member and Russia - warning Ankara that it will face economic sanctions under the Countering America's Adversaries Through Sanctions Act if it goes ahead with the purchase, reportedly costing more than $2bn.
So far, however, Ankara has refused to give in to US pressure, insisting that choosing which defence equipment to buy is a matter of national sovereignty.
The deal with Russia [aljazeera.com] has also raised concerns in Western circles that Turkey is drifting closer to Moscow's sphere of influence.
According to analysts, these purchases form more than just a military threat to the US.
They are about countering Russia's involvement in global conflicts, but also about maintaining long-standing US diplomatic relations and preventing Russia from receiving hard currency for its equipment, the analysts told Al Jazeera last year [aljazeera.com].