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.

Reaction from Washington, which has threatened to impose sanctions on Turkey, was limited, with acting Defense Secretary Mark Esper saying the U.S. position has not changed.

President Donald Trump gave a 30-minute speech on trade on Friday afternoon at a plant in Milwaukee run by a subsidiary of defense contractor Lockheed Martin but made no mention of Turkey.

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Washington has opposed Turkey's purchase, 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.

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

Defence Minister Hulusi Akar said three cargo planes had arrived, 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.

After speaking with his Esper, "Minister Akar told his U.S. counterpart that Turkey remains under a serious air and missile threat and that purchase of S-400 defense systems was not an option but rather a necessity," the source said about the phone call, adding that Ankara was still assessing acquiring U.S. Patriot missiles.

According to the Turkish defense ministry, Akar spoke with his U.S. counterpart in a phone call on Friday, and said Turkey remains under a serious air and missile threat and that purchase of S-400 defense systems was not an option but rather a necessity.

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.

After a meeting with President Donald Trump on the sideline of G20 summit in Japan last month, President Tayyip Erdogan said 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.

There has been bipartisan opposition in the U.S. Congress to Turkey's purchase and the reactions from lawmakers have been more stern, calling the State Department and Pentagon to remove Turkey from the F-35 program.

"President Erdogan was given a very clear choice. Unfortunately, he has clearly made the wrong one," a statement from House of Representatives Foreign Affairs Committee said.

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 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.

(Cover: First parts of a Russian S-400 missile defense system are unloaded from a Russian plane at Murted Airport, known as Akinci Air Base, near Ankara, Turkey, on July 12, 2019. /Reuters Photo)

(With input from Reuters)