For the third straight day Sunday, Turkey continued to receive components of the Russian-made S-400 missile defense system.

Delivery of the S-400 long-range air and missile defense system is continuing as planned, the Defense Ministry said. The ministry tweeted about the landing of a fifth, sixth and seventh Russian cargo aircraft at Ankara's Mürted Air Base, which was used by putschist soldiers during the foiled coup attempt that took place on July 15, 2016.

The ministry also said two more planes were expected to arrive within the day.

The long-awaited deliveries of the defense system began Friday when the first batch of equipment, procured to meet Turkey's air and missile defense needs, arrived at the air base in Ankara.

Three different Russian Antonov An-124 cargo aircraft landed on Friday and one arrived on Saturday. No details regarding when and where the Russian surface-to-air missiles will be installed have been revealed yet.

A Turkish military team has been training in Russia since May, and the training was initially planned to last five months. State broadcaster TRT said on Friday that the system would be assembled at Mürted by a joint Turkish-Russian technical team and installed in several locations across Turkey.

Citing diplomatic sources, the TASS Russian news agency reported on Friday that Russian air defense system will be delivered to Turkey in three installments, saying the second shipment of the S-400 will take place soon, while the third batch, which comprises of 120 missiles, will be delivered to Turkey by sea.

The Ankara-Moscow S-400 deal was inked in December 2017, when the parties signed a $2.5 billion agreement for two batteries of the S-400, which is Russia's most advanced long-range anti-aircraft missile system in use since 2007.

The 2017 contract with Russia followed lengthy efforts to purchase an air defense system from the U.S. with no success.

In recent months U.S. officials urged Turkey to buy U.S. Patriot missiles, arguing the Russian system would be incompatible with NATO systems and expose U.S. F-35 fighter jets to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance.

Turkey has urged the formation of a commission to clarify any technical issues, but the U.S. has failed to respond to this proposal.

The U.S. has also threatened sanctions over the S-400 purchase, with Turkey responding that any sanctions would be met in kind.