"A lot has been said and written about Turkey's purchase of the S-400 from Russia," Panayotopoulos said. "In my opinion, this raises concerns within NATO."
He believed that the concern of the United States and other NATO partners was justified, and added: "As far as I understand, the systems were not fully operated by Turkey. There was a series of experiments, but the system would be used at a later time."
Panayotopoulos said that "the extent of Turkey's safety in relation to the allies in NATO is now clear, and this is a matter of concern to all."
Turkey concluded a deal with Russia to purchase the Russian S-400 air defense system in April 2017, and received its first batch in July of last year, and conducted the first test of the system on October 16.