Turkey, a NATO ally of the U.S., will follow through with a deal to purchase anti-aircraft defenses from Russia, which could pose a significant international relations challenge to the Trump administration.

“Our President Recep Tayyip Erdogan has made it perfectly clear that the agreement between Turkey and Russia is final, and the issue of the S-400 purchase is closed,” Turkish Foreign Ministry spokesman Hami Aksoy told Russian-run media outlet TASS on Friday.