Turkey began taking the delivery of Russia’s S-400 air-defense system on Friday, completing a much-debated deal that is likely to trigger sanctions from the United States and test the NATO alliance.

The first components for the state-of-the-art system arrived aboard three Russian military planes at the Murted air base, located at a distant suburb of Ankara, the Turkish Defense Ministry said in a statement.

“Turkey received the first batch of S-400 air defense systems. The deliveries are sent to the Murted air base,” the ministry said. Two more deliveries are expected in the coming days.

Foreign Minister Mevlut Cavusoglu told reporters in Ankara that “there is no problem in the deliveries,” adding that “the process will also continue in a healthy pace in the future.”

The purchase, which is the fruit of a controversial agreement inked between Ankara and Moscow in 2017, signals, according to observers, Turkish President Recep Tayyip Erdogan’s willingness to coordinate more with Russia and could set off a new crisis in relations between Turkey and the U.S., two major NATO allies.

The U.S. President Donald Trump’s administration had given mixed signals about how it might respond if Turkey went through with the deal, but U.S. officials had warned of repercussions, including canceling sales of around 100 hi-tech U.S.-made F-35 fighter jets to Ankara and the imposition of sanctions under a 2017 law in cooperation with adversaries.

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