The North Atlantic Treaty Organization (NATO) Secretary General Jens Stoltenburg on Friday said that Turkey's decision to purchase missile defence system from Russia is a national decision.
"It is a challenge and is well known that there is a disagreement between Turkey and especially the United States on this issue," Anadolu Agency quoted Stoltenburg as saying at the conservative Heritage Foundation think tank in Washington D.C.
The NATO chief underscored that for NATO, which is an alliance of 29 North American and European countries, the most important thing is to see "that different systems can work together".
In December, Turkey signed an agreement with Russia that will see Moscow delivering the missile defence system to Ankara by early 2020.
In June, the U.S. Senate passed a bill prohibiting sales of F-35 jets to Turkey, citing the S-400 purchase.