Turkey’s pending purchase of the Russian S-400 missile defense system presents a national security problem for NATO, which would not be able to deploy F-35 aircraft alongside the Russian systems, senior U.S. officials said on Thursday…

NATO member Turkey has repeatedly said it is committed to buying the Russian missile defense system, despite warnings from the United States that the S-400s cannot be integrated into the NATO air defense system.

The U.S. State Department last week said Washington had told Turkey that if it buys the S-400 systems, the United States will have to reassess Ankara’s participation in the Lockheed Martin F-35 fighter program.