Russia has sent new components of its S-400 air-defense system to a military air base near the Turkish capital, Ankara.
Turkey's Defense Ministry tweeted on July 14 about the landing of a fifth and sixth Russian cargo planes at Murted air base, saying two more planes were expected within hours.
Turkey received the first three shipments of S-400 components on July 12 and a fourth shipment on July 13, defying threats of sanctions from the United States.
Washington and its allies consider the missile system to be incompatible with NATO systems and a threat to the F-35 fighter jets.
Washington has urged Turkey to purchase the U.S.-made Patriot missile system instead.
The United States has given Turkey a deadline of July 31 to reverse or face the loss of subcontracting work on the U.S.-led F-35 project.
But Washington has already started the process of removing Turkey from the F-35 program, halting training of Turkish pilots in the United States on the aircraft.
Based on reporting by AP and dpa