The U.S. State Department says it has warned Turkey over its potential purchase of a Russian air defense system.
State Department spokesman Robert Palladino said on 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 on 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.