The delivery process of Russian-made surface-to-air S-400 missile defense systems to Turkey has begun, the Defense Ministry said Friday.

The first batch of S-400 parts started arriving Friday at Mürted Air Base, formerly called the Akıncı Air Base, near capital Ankara, the ministry said in a statement.

Meanwhile, Russia’s federal service for military and technical cooperation confirmed the start of the deliveries of S-400 missile systems to Turkey, and said the delivery would be completed within the agreed time frame.

Ankara and Moscow inked a deal for the S-400s on April 11, 2017, when the parties signed a $2.5 billion agreement for two batteries of the systems — Russia’s most advanced long-range anti-aircraft missile system in use since 2007.

The United States has repeatedly warned Turkey it will face economic sanctions if it goes ahead with the purchase. It has also said Turkey won’t be allowed to participate in the program to produce the high-tech F-35 fighter jets.

Turkey has refused to bow to U.S. pressure, insisting that choosing which equipment to purchase is a matter of national sovereignty.