Excluding NATO-member Turkey from the trillion-dollar F-35 fighter jet program would be challenging due to Ankara’s integral role in the stealthy jet’s production process, but not impossible, U.S. sources familiar with the situation said.

Last week Reuters reported that the United States could soon freeze preparations for delivering F-35 fighter jets to Turkey, a move that would widen the rift between Ankara and Washington, the latest disagreement in a years-long standoff.

At the heart of the matter lies Turkish President Tayyip Erdogan’s commitment to buy a Russian air defense system that the United States says would compromise the security of F-35 aircraft, which is built by Lockheed Martin Corp.

The United States and other NATO allies who own F-35 fighter jets fear the radar on the Russian S-400 missile system will learn how to spot and track the F-35, making it less able to evade Russian weapons in the future.

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