"If Turkey takes the S-400, there would be grave consequence in terms of our military relationship, and the Patriots and the F-35s," Summers was quoted as saying by Defense News. "They will not get the F-35s if they take the S-400."
Summers said the United States may also block Ankara from purchasing Patriot missile systems.
On Thursday, President Recep Tayyip Erdogan reaffirmed that the deal with Russia on deliveries of the S-400 air defence systems was completed, and said a new deal on purchases of advanced Russian-made S-500 systems could be possible in future.
READ MORE: S-400 Purchase Doesn't Mean Ankara Has Rejected US' Patriots — Turkish VP
In January, Turkish Foreign Minister Mevlut Cavusoglu said that Ankara was ready to consider the US proposal on deliveries of the Patriot air defence systems, but refused to abandon the S-400 missile systems deal with Russia as a potential precondition for the Ankara-Washington agreement.
