"It is a challenge and is well known that there is a disagreement between Turkey and especially United States on this issue," Stoltenberg said at the conservative Heritage Foundation think tank in Washington D.C.

The NATO chief mentioned that for NATO the most important thing is "the different systems can work together."

Last December, Turkey announced it concluded an agreement with Russia for the purchase of two S-400 systems 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.

Stoltenburg went on to discuss the role Turkey plays as a part of NATO and how important of an ally the country is in terms of its geographic location, its contribution to the fight against the Islamic State (IS) and its role hosting Syrian refugees.

"Turkey is also important when it comes to dealing with the migrant and refugee crisis. They host millions of refugees, and they are important to implement the agreement between EU and Turkey on managing the flow of migrants of the Aegean sea," Stoltenburg said.

He said Turkey has suffered by far the highest number of terrorist attacks, as well as a failed coup attempt.

"So, yes there is problem, there is a challenge, with the decision to buy S-400 combined with F-35. I welcome the dialogue that is addressed, but at the same time it’s important to recognize the importance that Turkey is playing to the whole alliance," Stoltenburg added.

Reporter’s code: 50101