U.S. officials are worried the Turkey-Russia deal could give the Kremlin access to crucial intelligence shared between Washington. and Turkey and compromise NATO developments.
Experts are especially concerned about the development of the F-35 fighter jet, with which Turkey is involved. Officials told Turkey if the S-400 purchase went through the country would be barred from the F-35 program.
The U.S. has strongly urged NATO member Turkey to pull back from the deal -- reportedly costing more than $2 billion -- warning the country that it will face economic sanctions under the Countering America's Adversaries Through Sanctions Act if it goes ahead with the purchase. It has also said Turkey won't be allowed to participate in the program to produce high-tech F-35 fighter jets.
Sanctions would mark a new low in the already-tense relations between Turkey and the U.S. Last year, the United States imposed sanctions on Turkey over its detention of an American pastor, triggering a Turkish currency crisis.
The deal with Russia -- the first such deal between Russia and a NATO member -- has also raised concerns that Turkey is drifting closer to Moscow's sphere of influence. Turkey has refused to bow to U.S. pressure, insisting that choosing which defense equipment to purchase is a matter of national sovereignty. It has said it was forced to buy the S-400s because Washington refused to supply the American-made Patriot systems to Turkey.
U.S. officials have since encouraged Turkey to buy the Patriot missile defense system instead of the S-400s. But Turkey says the offer does not meet its requirements, including possible future joint production.
The U.S. has already stopped training Turkish pilots on the F-35, and given Ankara until the end of July to get its personnel out of the U.S.
Turkey maintains that it has fulfilled all of its financial obligations concerning the F-35 program and cannot be excluded from the project.
The Associated Press and Fox News' Hollie McKay contributed to this report.