"We have started executing the contract [for S-400 deliveries to Turkey]," Mikheev said, explaining that production of the missile systems had started, but are not yet being supplied.
Russian-Turkish cooperation on S-400 deliveries has been repeatedly criticized by NATO, which has stated that integration between the different air defense systems will be impossible.
The US State Department also raised concerns about the issue, warning Turkey of possible sanctions over the purchase of the Russian missile systems.
"Ankara claims to have agreed to purchase the Russian S-400 missile system, which could potentially lead to sanctions under section 231 of CAATSA and adversely impact Turkey’s participation in the F-35 program," Assistant Secretary of State Wess Mitchell told the House Foreign Affairs Committee.
Russia and Turkey signed a loan agreement for the supply of S-400 systems to Ankara in December 2017. According to Turkish officials, their country will acquire two S-400 batteries, which are to be operated by Turkish military personnel.