The U.S. State Department said it had warned Turkey that if it goes ahead with the purchase of a Russian air defense system.

State Department spokesman Robert Palladino said March 5 that Washington might reconsider giving Turkey access to the advanced F-35 fighter jet program if it buys the Russian S-400 system.

“We’ve clearly warned Turkey that its potential acquisition of the S-400 will result in a reassessment of Turkey’s participation in the F-35 program and risk other potential future arm transfers to Turkey,” Palladino said.

The comments were echoed earlier March 5 by the head of U.S. European Command, General Curtis Scaparrotti, in comments to the Senate Armed Services Committee.

“It presents a problem to all of our aircraft, but specifically the F-35,” Scaparrotti told the committee.

The U.S. had agreed to sell 100 of its latest, fifth-generation F-35 fighters to Turkey, but has so far delivered only two of the aircraft.

Congress last year ordered a delay in future deliveries.

Washington has also said it could withdraw an offer to sell Turkey the U.S. missile defense equivalent: the Patriot anti-missile defense system.

Russia media reported last month that Turkey intended to buy four S-400 units for a price of $2.5 billion.

NATO and U.S. officials fear that Turkey’s acquisition of the Russian system could allow it to gain access to classified NATO technologies.