Ankara has gone ahead with its purchase of the Russian defence system despite threats of US sanctions.

Turkey will receive the second batch of the Russian S-400 missile system on Tuesday, Minister of Defence Hulusi Akar has said.

Ankara received its first supply of S-400 missiles in July, despite a warning by the United States about possible sanctions. The acquisition of the highly-advanced air defence system has led to a standoff between Turkey and its NATO allies, especially the US.

Deliveries of the system are set to continue until April 2020.

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.

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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 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.

Sanctions would mark a new low in the already tense relations between Turkey and the US.

Last year, the US imposed sanctions on Turkey over the detention of an American pastor, triggering a Turkish currency crisis. The sanctions were later lifted upon the pastor’s release.

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The deal with Russia 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.