A spokeswoman for the US State Department said Morgan Ortigas told reporters that "Turkey will subject itself to real and negative consequences if accepted as system -400."
In early June, the United States granted Turkey a deadline in late July to stop buying the S-400 anti-missile system because it runs counter to Washington's view of its new stealth F-35, which Turkey wants to buy as well.
If Turkey does not give up the purchase of the S-400 by July 31, the Turkish pilots currently training in the United States on F-35 jets will be expelled, and contracts awarded to Turkish companies to cut off F-35s will also be canceled, the Pentagon said.
The Defense Department justified the warning that Turkey had sent troops to Russia to train on the use of S-400 missiles.
According to the Pentagon, Washington will also not hand over to Ankara four F-35s it bought but is still in US territory.
Despite the US threats, Turkey has so far continued its deal with Russia on the S-400 missiles.