"Turkey, a country of 80 million people, is an important NATO ally. Today's news that Turkey has taken delivery of the first batch of S-400 air defense systems from Russia is not good," Paet wrote on his social media page Friday, BNS reports.

The S-400 anti-aircraft weapons system (NATO codename: Growler) was developed in the 1990s as an upgrade to the earlier S-300 family of weapons, entering service with the Russian military over 10 years ago. Described by some experts as one of the best, if not the best, air defense system currently available, it is capable of destroying aircraft, cruise and ballistic missiles and even ground targets. One system can control up to a reported 72 launchers, with a total of close to 400 missiles.

"Turkey's move to buy the systems from Russia has caused tensions in relations with the U.S. and other NATO allies. The question is both about political choices and the compatibility of weapons systems," he continued.

NATO itself has expressed concerns about the move as well; the S-400 systems could compromise the alliance's own set-up, most notably the Lockheed Martin F-35 Lightning II stealth fighter (S-400 systems include high-altitude detection equipment).

Meeting with think tank representatives at the Pentagon earlier this week, Katie Wheelbarger, acting assistant secretary of defense for international security affairs, said the NATO alliance and other nations partnering in developing and fielding the F-35 are unified in their opposition to Turkey's use of S-400. The system could even be used to gather data on the F-35's capabilities, information which could ultimately end up in Russian hands, officials explained, according to BNS.

Turkey's quest to obtain S-400 systems stretches back around 10 years; the U.S. had previously threatened sanctions if the country pressed ahead with its procurement. Current operators include China, which started inducting S-400 systems early on this year, with India and Saudia Arabia likely future purchasers.

Turkey has been a NATO member almost as long as the alliance has existed, being the first expansion nation (together with Greece) in 1952, just three years after NATO's formation. Estonia joined in 2004.