ANKARA, Turkey (AP) — The Latest on Russian missile systems arriving in Turkey (all times local):

6:55 p.m.

The acting U.S. secretary of defense says Washington is aware that Turkey has begun taking delivery of a Russian-made S-400 air defense system.

Mark Esper, who is expected to be officially nominated next week to be defense secretary, told reporters at the Pentagon that the U.S. remains unwilling to allow Turkey to acquire the U.S. F-35 stealth fighter, as long as it has the S-400.

Esper said he planned to speak to his Turkish counterpart later in the day, and that the Pentagon would have more to say about the dispute afterward.

The Trump administration has repeatedly told Turkey that it will be cut off from the F-35 fighter program if it buys Russian air defenses, because the S-400 is incompatible with NATO defenses and could jeopardize sensitive information about F-35 technologies.

10:50 a.m.

Turkey's Defense Ministry says the first shipment of a Russian missile defense system has arrived in Turkey, a development that could move the country closer to U.S. sanctions.

The ministry said the first delivery of the S-400 air defense systems arrived on Friday at an air base near the capital, Ankara.

The United States has warned Turkey it will face economic sanctions if it goes ahead with the purchase of a Russian missile defense system. 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.