The purchase of Russia's S-400 air defense systems by Turkey is a "sovereign" decision of the government in Ankara, however, Turkey must take into account that these weapons will not be compliant with NATO systems, US State Secretary Jim Mattis told reporters on Monday.

Axar.az reports citing Sputnik.

In September, the press service of Russia's Federal Service for Military-Technical Cooperation (FSMTC) said that Moscow and Ankara had reached an agreement on the delivery of S-400 systems to Turkey. Erdogan said that Turkey had already put down a deposit for the S-400 systems. On September 29, Russian presidential aide Vladimir Kozhin confirmed that Turkey had made an up-front payment for the S-400s, adding that the delivery is scheduled for 2019.

In late October, NATO’s Military Committee Chairman Gen. Petr Pavel warned Turkey that there may be consequences if the country procured Russia's S-400 air defense system, adding that it will prevent Turkey from becoming part of any integrated air defense system.

On Monday, US Secretary of Defense Jim Mattis also warned Turkey about the pitfalls of the decision.

"That's a sovereign decision for Turkey. Clearly, it will not be interoperable with NATO. So they're going to have to consider that if they go forward," Mattis.

The S-400 Triumph is Russia's next-generation mobile surface-to-air missile system carrying three different types of missiles capable of destroying aerial targets at a short-to-extremely-long range. It integrates multifunctional radar, autonomous detection and targeting systems, anti-aircraft missile systems, launchers, and a command and control center.