US Secretary of Defense raises concerns about 'interoperable' Russian missile defense system.

U.S. Secretary of Defense James Mattis has raised concerns over Turkey’s purchase of the S-400 air-defense system from Russia, saying it is “not interoperable with NATO.”

Addressing journalists at the U.S. Department of Defense, Mattis referred to the Turkey-Russia S-400 deal, saying: “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,” he added.

Mattis: S-400 deal a 'sovereign decision' for Turkey
Turkey has completed the purchase of the S-400 air-defense system from Russia, National Defense Minister Nurettin Canikli said on Saturday.

The S-400 is Russia’s most advanced long-range anti-aircraft missile system and can carry three types of missiles capable of destroying targets, including ballistic and cruise missiles.

It can track and engage up to 300 targets at a time and has an altitude ceiling of 27 kilometers (17 miles).