Russia began delivery of an advanced missile defence system to Turkey on Friday, a move expected to trigger US sanctions against a NATO ally and drive a wedge into the heart of the Western military alliance.
The first parts of the S-400 air defence system were flown to a military air base near the capital Ankara, the Turkish Defence Ministry said, sealing Turkey's deal with Russia which Washington had struggled for months to prevent.
The United States says the purchase of Russian military hardware may lead to Ankara's expulsion from an F-35 fighter jet programme.
NATO said on Friday it was "concerned" by Turkey's acquisition.
"We are concerned about the potential consequences of Turkey's decision to acquire the S-400 system," a NATO official told AFP, following repeated warnings that the system is not compatible with allied weapons.
Investors in Turkey have been unsettled by the deal. The Turkish lira weakened to 5.717 against the dollar from 5.683 before the ministry announced the arrival of the S-400 consignment to the Murted Air Base, northwest of Ankara.
“The delivery of parts belonging to the system will continue in the coming days,” Turkey's Defence Industry Directorate said. “Once the system is completely ready, it will begin to be used in a way determined by the relevant authorities.”
At least two Russian Air Force AN-124 cargo planes flew to Turkey on Friday morning, data from plane tracking website Flightradar24 showed. Turkish broadcasters showed footage of one plane parked at airbase and a second one landing at around 12.30 pm (0930 GMT).
Russia's Federal Service for Military-Technical Cooperation confirmed on Friday it had started delivering the S-400 systems and that the deliveries would continue in accordance with an agreed schedule, the RIA news agency reported.
Turkey says the system is a strategic defence requirement, particularly to secure its southern borders with Syria and Iraq. It says that when it made the deal with Russia for the S-400s, the United States and Europe had not presented a viable alternative.
President Tayyip Erdogan said after meeting President Donald Trump at a G20 summit last month that the United States did not plan to impose sanctions on Ankara for buying the S-400s.
Trump said Turkey had not been treated fairly but did not rule out sanctions. U.S. officials said last week the administration still plans to impose sanctions on Turkey.
Under legislation known as Countering America's Adversaries through Sanctions Act (CAATSA), which targets purchases of military equipment from Russia, Trump should select five of 12 possible measures.
These range from banning visas and denying access to the U.S.-based Export-Import Bank, to the harsher options of blocking transactions with the U.S. financial system and denying export licences.
Washington says the S-400s could compromise its Lockheed Martin F-35 stealth fighter jets, an aircraft Turkey is helping to build and planning to buy.
Turkey could also face expulsion from the F-35 programme under the sanctions. Erdogan has dismissed that possibility, but Washington has already started the process of removing Turkey from the programme, halting training of Turkish pilots in the United States on the aircraft.
Investors in Turkey have been concerned about the impact of potential U.S. sanctions on an economy which fell into recession after a currency crisis last year.
The S-400 acquisition is one of several issues which have frayed ties between the two allies, including a dispute over strategy in Syria east of the Euphrates River, where the United States is allied with Kurdish forces that Turkey views as foes.
The detention of U.S. consular staff in Turkey has also strained relations, along with disagreements over Iran, Venezuela and Middle East policy. Turkey has long demanded Washington hand over a Muslim cleric which Ankara holds responsible for an attempted coup in 2016.
The Murted base was formerly known as Akinci Air Base and was used by putschist soldiers in the failed 2016 coup