© Reuters. A Russian plane carrying first parts of a Russian S-400 missile defense systemÊlands Murted Airport near Ankara

By Sarah Dadouch and Ezgi Erkoyun

ISTANBUL (Reuters) - Turkey began taking delivery of an advanced Russian missile defense system on Friday, a move expected to trigger U.S. 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 defense system were flown to the Murted military air base northwest of Ankara, the Turkish Defense Ministry said, sealing Turkey's deal with Russia which Washington had struggled for months to prevent.

In the first comments from Washington, acting Defense Secretary Mark Esper said the U.S. position on the issue has not changed. The United States has threatened to impose sanctions on Ankara, saying Russian military hardware is not compatible with NATO systems and that the acquisition may lead to Turkey's expulsion from an F-35 fighter jet program.

Investors in Turkey have been unsettled by the deal. The Turkish lira weakened as much as 1.6% to 5.7780 against the dollar, before recovering somewhat, after the ministry announced the arrival of the S-400 consignment. The main Istanbul share index fell 2.13%.

Turkish broadcasters showed footage of huge Russian Air Force AN-124 cargo planes offloading equipment at the air base.

"Today three cargo planes arrived," Defense Minister Hulusi Akar told state-owned Anadolu news agency, adding that deliveries would continue in coming days.

A second delivery by air will take place soon, Russia's TASS news agency quoted an unnamed military-diplomatic source as saying. A third delivery – of 120 guided missiles – will be carried out by ship at the end of the summer, the source said.

Twenty Turkish servicemen received training from Russia in May-June and 80 more Turkish servicemen will receive training to use the S-400 system, the source was quoted as saying.

"We are aware of Turkey taking delivery of the S-400, our position regarding the F-35 has not changed and I will speak with my Turkish counterpart Minister Akar this afternoon," Esper said. "There will be more to follow after that conversation," he said.

ERDOGAN-TRUMP TALKS

Turkey says the system is a strategic defense 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, and U.S. officials said last week the administration still plans to act.

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 licenses.

Washington says the S-400s could compromise its Lockheed Martin (NYSE: ) 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 program under the sanctions. Erdogan has dismissed that possibility, but Washington has already started the process of removing Turkey from the program, 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.

Turkey's dollar bonds dropped to three-week lows on the news of the delivery, while the cost of insuring exposure to Turkish sovereign debt also rose.

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.