NATO Secretary General Jens Stoltenberg said Thursday that purchasing the S-400 missile defense system from Russia is Turkey's decision.
Stoltenberg's comments came amid a debate between Turkey and several NATO members whether the system will be compatible with the security bloc.
President Recep Tayyip Erdoğan dismissed Wednesday concerns over the agreement, while criticizing the stance of those criticizing the deal.
"They went crazy because we made the S-400 agreement. What were we supposed to do, wait for you? We are taking and will take all our measures on the security front," Erdoğan said during a meeting with mayors from the ruling Justice and Development Party (AK Party) in Ankara.
Turkey has agreed to pay Russia $2.5 billion for the S-400 systems, a Turkish official said on July 13.
Turkey said in April that NATO ally nations had not presented a "financially effective" offer on alternative missile defense systems.
The decision to procure the Russian system comes as Turkey finds itself frequently at odds with NATO allies, particularly the United States and Germany. Ankara has been angered by U.S. support for the PKK's Syrian offshoot YPG in the battle against Daesh in Syria.
The U.S. Pentagon said it had expressed concerns to Turkey about the deal.
"We have relayed our concerns to Turkish officials regarding the potential purchase of the S-400. A NATO interoperable missile defense system remains the best option to defend Turkey from the full range of threats in its region," spokesman Johnny Michael said in a statement.
Turkey's need for an air missile defense system once again became urgent with the start of the civil war in Syria, a country which has a sizeable ballistic missile stockpile able to strike a large portion of Turkey's territory, in addition to biological and chemical weapons.
Turkish officials are concerned that the missiles could be used by the Assad regime in case of a unilateral military conflict or an international military intervention, while the possibility of the missiles falling into the hands of a terrorist group poses greater danger.
The country had to rely on Patriot batteries provided by its NATO allies Germany, the Netherlands and Spain during the conflict, which were withdrawn in 2015.
Turkey faced similar worries during the Iran-Iraq War and the First and Second Gulf Wars due to the ballistic missiles possessed by those countries.
In 2013, Turkey selected China Precision Machinery Import-Export Corporation for a long-range air defense system but later scrapped the deal under pressure from NATO.
In recent years, Turkey has sought to build its domestic defense industry to reduce reliance on foreign suppliers.
Introduced in 2007, the S-400 is Russia's most advanced long-range anti-aircraft missile system and can carry three types of missiles capable of destroying targets including ballistic and cruise missiles.
It can track and engage up to 300 targets at the same time and has an altitude ceiling of 27 kilometers (17 miles).
Last year, both China and India signed an intergovernmental agreement for the procurement of four regiments of Russian-made S-400s and delivery of the new air defense systems is expected to begin in 2020.
The defense system is able to carry missiles, which has the capability to destroy ground and air targets, including ballistic and cruise missiles.
