“It is a challenge and is well-known that there is a disagreement between Turkey and especially [the] United States on this issue,” Stoltenberg said at the conservative Heritage Foundation think tank in Washington D.C.
He added that for NATO the most important thing is that “different systems can work together.”
Business and financial television news network CNBC, citing an unnamed informed source, reported last week that Turkey has begun building a site for the S-400 missile system.
The report, however, gave no indication about where the site is located in Turkey.
A number of NATO member states have criticized Turkey for its planned purchase of the S-400 missile defense systems from Russia, stating that the move could jeopardize Ankara's acquisition of F-35 fighter jets.
On July 15, the top American Air Force general in Europe warned that the Turkish government’s plan to purchase Russian S-400 systems would give the advanced air defense shield deep insight into the US radar-evading F-35 fighter jets.
“Anything that an S-400 can do that affords it the ability to better understand a capability like the F-35 is certainly not to the advantage of the coalition,” said General Tod Wolters, who is also the NATO Allied Air Commander.
Turkish President Recep Tayyip Erdogan has emphasized that Ankara would utilize sophisticated S-400 anti-aircraft missile systems if necessary.
“We will not just buy the S-400s and place them in a storehouse. We will use them if need be. These are defense systems. What are we going to do with them if not use these defense systems? Are we going to depend on the United States again? We have been demanding them for years, but the answer given to us has been: The [US] Congress is not allowing. We are tired of this,” he said in remarks broadcast live by private Turkish-language TGRT Haber television news network on June 12.
The S-400 system, whose full name is the Triumf Mobile Multiple Anti-Aircraft Missile System (AAMS), is an advanced Russian missile system designed to detect, track, and destroy planes, drones, or missiles as far as 402 kilometers away. It has previously been sold only to China and India.
Turkey is striving to boost its air defense, particularly after Washington decided in 2015 to withdraw its Patriot surface-to-air missile system from Turkey's border with Syria, a move that weakened Turkey’s air defense.
Before gravitating towards Russia, the Turkish military reportedly walked out of a $3.4 billion contract for a similar Chinese system. The withdrawal took place under purported pressure from Washington.
Ankara’s ties with its Western allies in NATO have been strained over a range of issues.
Erdogan has been critical of Washington for supporting Kurdish groups in Syria that he says are responsible for terror attacks inside Turkey.
The Turkish leader has also slammed American officials for rejecting his requests to hand over Fethullah Gulen, a powerful opposition figure living in the US, whom Ankara accuses of having masterminded the July 2016 coup attempt.