Turkey, an F-35 program partner, is currently slated to receive two of the jets. That delivery of Lockheed Martin's fifth-generation jets is the start of what Ankara hopes will eventually amount to 100 of the stealth aircraft.
In June, the U.S. defense giant held a formal hand-off ceremony at its F-35 facility in Fort Worth, Texas. After the ceremony, Lockheed ferried the aircraft to Luke Air Force Base in Arizona where Turkish pilots began training alongside U.S. airmen.
The Russian-made S-400 missile system, which is equipped with eight launchers and 32 missiles, is capable of targeting stealth warplanes like the F-35 fighter.
In the colossal $717 billion National Defense Authorization Act, Congress tasked the Pentagon to deliver a report in 90 days outlining the potential risks associated with Turkey's purchase of the S-400 missile system.
"We are going through this current issue between us, and we are engaged in, I would call it, frequent, right now, very frequent, discussions at very high levels to try to sort this out," Defense Secretary James Mattis said Tuesday when asked about the F-35 sale to Turkey.
"I believe that there is sincerity on both sides to try to work this out. So we're engaged in it right now, and I — you know, I need to work with them directly on this, as does Secretary (of State Mike) Pompeo and others on our side," he added.
What's more, tensions between the U.S. and Ankara have intensified over the detention of American pastor Andrew Brunson.