The amendment to the National Defense Authorization Act, or NDAA, from Democratic Senator Jeanne Shaheen and Republican Senator Thom Tillis, would remove Turkey from the F-35 program over its detention of U.S. citizen Andrew Brunson, Shaheen’s office said.

It also faults NATO ally Turkey for its agreement with Russia in December 2017 to buy S-400 surface-to-air missile batteries. Ankara wants the system to boost its defense capabilities amid conflicts across its borders in Syria and Iraq.

According to Shaheen’s office, the intention to purchase the Russian system is sanctionable under U.S. law, Hurriyet daily reported.

“There is tremendous hesitancy [about] transferring sensitive F-35 planes and technology to a nation who has purchased a Russian air defense system designed to shoot these very planes down,” said Senator Shaheen.

Relations between Ankara and Washington have been strained over a host of issues in recent months, including U.S. policy in Syria and a number of legal cases against Turkish and U.S. nationals being held in the two countries.

Turkey has said it would retaliate if the U.S. enacted a law halting weapons sales to the country.

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