WASHINGTON (Reuters) - U.S. Defense Secretary James Mattis said on Tuesday the United States was concerned about Turkey’s purchase of a Russian missile defense system that cannot be integrated into NATO.

The United States has already warned Turkey that it would be subject to sanctions and its purchase of Lockheed Martin fighter jets would be jeopardized if Ankara does not drop plans to purchase the missile defense system.

“Clearly, Turkey bringing a Russian anti-aircraft, anti-missile system into a NATO country -- we cannot integrate that into NATO. Yes, it does concern us and we do not recommend that,” Mattis told reporters.