Turkey's Defense Ministry says the first parts of the S-400 Russian missile defense systems have delivered to Ankara and deliveries will continue in the coming days.
Ankara's deal with Moscow has been a major source of tension between Turkey and Washington.
The S-400 consignment was delivered on July 12 to the Murted air base outside the capital Ankara, the ministry said, in a statement.
The announcement immediately triggered a weakening in the Turkish lira to 5.7 against the dollar from 5.6775 on July 12.
"The delivery of parts belonging to the system will continue in the coming days," Turkey's Defense Industry Directorate said separately.
"Once the system is completely ready, it will begin to be used in a way determined by the relevant authorities."
Russia's Federal Service for Military and Technical Cooperation confirmed the start of the deliveries, while Kremlin spokesman Dmitry Peskov told reporters on July 12 that "everything is being done in strict accordance with the two countries' agreements," and that "the parties are fulfilling their obligations."
The Pentagon is scheduled to hold a press briefing on July 12 to outline its response to "Turkey accepting delivery" of the S-400 system, it said in a statement.
The United States has said that if fellow NATO member Turkey does not cancel the S-400 deal by July 31, Ankara will be blocked from purchasing the next-generation F-35 fighter jets.
Washington has urged Turkey to purchase the U.S.-made Patriot missile system instead.
Speaking before opening bilateral talks with Uzbekistan at the Pentagon on July 12, acting U.S. Defense Secretary Mark Esper said that "our position regarding the F-35 has not changed," adding that he would speak with Turkish Defense Minister Hulusi Akar later in the day.
NATO has yet to react officially to the Turkish announcement, but an alliance official speaking on condition of anonymity told the AFP news agency that the 29-member bloc is "concerned about the potential consequences" of the purchase.
U.S. President Donald Trump met with Erdogan on the sidelines of last month's G20 summit in Osaka, urging him not to proceed with the purchase of Russia's advanced S-400 air-defense system.
Erdogan told Trump during their meeting on the margins of the G20 meeting in Japan that former U.S. President Barack Obama did not allow Ankara to buy Patriot missiles, an equivalent of the S-400s.
Washington has already started the process of removing Turkey from the F-35 program, halting training of Turkish pilots in the United States on the aircraft.
Ankara plans to buy 100 of the jets for its own military's use.