Turkey received the first batch of Russia's S-400 missile defense system on Friday, sparking NATO "concern" and risking deepening tensions with the United States, which has repeatedly warned against the purchase.
The delivery to an airbase in the Turkish capital Ankara comes after Washington warned this week that there would be "real and negative" consequences if Turkey bought the defense system.
NATO, which counts Turkey as one of its members, is "concerned about the potential consequences" of the purchase, an official told AFP.
The alliance has repeatedly warned Turkey that the Russian system is incompatible with other NATO weapons systems, not least the F-35, a new generation multi-role stealth fighter jet.
The US fears that if Ankara integrates the S-400 into its defenses there is a risk that sensitive data about its F-35 could leak back to the Russians and it has threatened to expel Turkey from its fighter jet program.
But Turkish President Recep Tayyip Erdogan has refused to back down and said he is confident Turkey will not face US sanctions.
On Friday, the Turkish defense ministry issued a statement confirming "the delivery of the first shipment of parts of the S-400 long-range regional air missile defense system" had begun.
It was not immediately known where the defense system would be deployed or when it would be operational.
Turkey's Presidency of Defence Industries said in a statement that the delivery of the system's other parts would continue "in the coming days".
Kremlin spokesman Dmitry Peskov told journalists on Friday that "everything is happening in strict accordance with the agreements and signed contracts, all obligations are being carried out".
- Potential US sanctions -
The US State Department has said Turkish officials are fully aware of legislation -- the Countering America's Adversaries Through Sanctions Act -- which mandates sanctions for any "significant" purchases of weapons from Russia.
Washington has given Ankara until July 31 to cancel the S-400 purchase or have its pilots kicked off its F-35 training course and expelled from the US.
US President Donald Trump's pick for Pentagon chief, Mark Esper, confronted the Turkish defense minister about the deal on the sidelines of a NATO meeting last month.
The first parts of the Russian missile system arrived on three planes at Ankara's Murted airbase, Turkish media reported. It arrived three days before the anniversary of a failed 2016 coup attempt.
- 'Game-changer'-
Nicholas Danforth, of the German Marshall Fund of the United States, told AFP the delivery reflected Ankara's desire to forge "a more independent foreign policy and reset the terms of its relationship with the US".
He said Turkish policymakers believe that "more assertive moves will ultimately force Washington to be more accommodating of their interests".
Turkey has repeatedly refused to back down from the agreement with Russia, despite US threats.
"We say this each time. This is a done deal. The process continues. We are coordinating this work, whether permission for planes, personnel," Foreign Minister Mevlut Cavusoglu told reporters in Ankara on Friday.
US support for Kurdish militia in Syria deemed as "terrorists" by Ankara is one of the major stumbling blocks in relations between the two countries, as well as Washington's refusal to extradite Muslim preacher Fethullah Gulen blamed for the coup bid.
Nick Heras, of the Center for a New American Security, said the S-400 system would be a "game changer" for Turkey's air defense strategy in a region surrounded by actors with well-developed air forces.
"It is no secret that Erdogan is positioning Turkey to be a 'Eurasian' power, which means that Turkey needs to balance its relationship with China and Russia as much as it does with the United States and NATO," he told AFP.
"Turkey is not guaranteed to be in the American camp forever."
