NATO on Thursday said its allies can decide on what equipment they purchase, stressing that the "interoperability" of its component armies is key.
"It is up to Allies to decide what military equipment they buy. What matters for NATO is that the equipment Allies acquire is able to operate together," an emailed NATO statement to Anadolu Agency read.
"Interoperability of our armed forces is fundamental to NATO for the conduct of our operations and missions," it added.
The NATO comments come after President Recep Tayyip Erdoğan's statement earlier this week that a first payment for the S-400 air defense system had been made to supplier Russia.
The deal has caused concern among Turkey's NATO allies as the Russian system is incompatible with the air defenses of other NATO members.
Turkish officials, however, are insisting the system will be compatible with NATO technology.
Erdogan also said: "Greece, which is also a NATO country, has been using S-400 for years. Why did they [NATO] not raise their voice on it?"
However, Wednesday's statement denied the Turkish president's claim, saying: "No NATO ally currently operates the S-400."
The statement added that NATO "has not been informed about the details of any purchase".
