ISTANBUL (AP) — For the third straight day, Russia has delivered components of its S-400 air defense system to Turkey.

Turkey's Defense Ministry tweeted Sunday about the landing of a fifth and sixth Russian cargo planes at Murted Air Base, near the capital, Ankara. It said two more planes were expected within hours.

The deliveries for the Russian-made defense system, which is driving a rift between Turkey and the United States, began Friday.

The United States has repeatedly warned it will impose economic sanctions and kick Turkey out of the F-35 stealth fighter jet program if Ankara does not drop its S-400 purchase.

Turkey, a fellow NATO nation, has refused to bow to U.S. pressure, saying its defense purchase is a matter of national sovereignty.