Turkey continues to receive components of Russian S-400 missile defense systems at the Murted Air Base in Ankara, Anadolu Agency reports.

Delivery of the long-range air defense systems continued, with 14 shipments have so far landed in Turkey over last six days.

Following protracted efforts to purchase an air defense system from the U.S. with no success, Ankara signed a contract in 2017 to purchase the S-400s from Russia.

U.S. officials urged Turkey to buy U.S. Patriot missiles, arguing the Russian system would be incompatible with NATO systems and expose the F-35s to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance. It has urged the formation of a commission to clarify any technical issues, but the U.S. has failed to respond to Turkey’s proposal.

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The U.S. threatened sanctions over the purchase, with Turkey responding that any sanctions would be met in kind.