Turkey received the first batch of Russia's S-400 missile defence system on Friday in a move expected to raise tensions with the United States, which has repeatedly warned against the purchase.
The delivery at an air base in the Turkish capital Ankara comes after Washington warned this week that there would be "real and negative" consequences if Turkey bought the defence system.
"The delivery of the first shipment of parts of the S-400 long range regional air missile defence system began as of July 12, 2019 to Murted air base in Ankara," Turkey's defence ministry said in a statement.
It was not immediately known where the defence system would be deployed or when it would be operational.
"It will be operational in a manner determined by relevant authorities once the system is entirely ready," Turkey's Presidency of Defence Industries (SSB) said in a statement, adding that the delivery of the system's other parts would continue "in the coming days".
Russia's Federal Service for Military-Technical Cooperation confirmed the delivery to the TASS news agency.
TASS also quoted a Russian military-diplomatic source as saying that another plane carrying S-400 parts will depart for Turkey "in the near future", while a third delivery consisting of 120 guided missiles will be shipped by sea "most likely at the end of the summer".
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 the Countering America's Adversaries Through Sanctions Act, a law passed by Congress in 2017 that mandates sanctions for any "significant" purchases of weapons from Russia.
Washington has threatened to remove Turkey from its F-35 fighter jet programme, giving Ankara until July 31 to cancel the S-400 purchase or have its pilots kicked off the training course and expelled from the US.
But Turkish President Recep Tayyip Erdogan said after meeting US counterpart Donald Trump last month that he was confident Ankara would not face sanctions for buying the Russian missile system.
Erdogan told Trump during their meeting on the margins of the G-20 meeting in Japan last month that former US president Barack Obama did not allow Ankara to buy Patriot missiles -- an equivalent of the S-400s.
Trump appeared to be convinced, saying: "You can't do business that way. It's not good."
The first parts of the Russian missile system arrived on two planes at Ankara's Murted air base, Turkish media reported. Turkey's air force changed the name of the base from Akinci to Murted after it was at the centre of a 2016 failed coup.
The delivery comes two days after US ambassador-designate to Turkey, David Satterfield, arrived in Ankara.
But despite the US threats, Turkish officials repeatedly insisted the agreement with Russia was a "done deal".
"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.
"There are no problems, the process will continue in a healthy manner."
The US and NATO have said that the S-400 is incompatible with equipment used by other members of the alliance.
Nick Heras, of the Center for a New American Security, said the S-400 system would be a "game changer" for Turkey's air defence strategy in 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."