The United States has given Turkey a deadline of July 31 to reverse its purchase of a Russian missile-defense system or face the loss of subcontracting work on the U.S.-led F-35 fighter jet project.
Ellen Lord, U.S. undersecretary of defense for acquisition and sustainment, also told reporters on June 8 that Turkish pilots training in the United States on F-35 warplanes being sold to Ankara will be expelled if the missile deal is not called off.
She said the deadline "will allow sufficient time for Turkish personnel associated with the F-35 program to be reassigned and depart the United States...to facilitate an orderly cessation of Turkish participation."
Acting U.S. Defense Secretary Patrick Shanahan sent a letter to his Turkish counterpart that explained the measures to remove Turkey from the program unless Ankara changes course.
Washington and its allies consider Russia’s S-400 missile system to be incompatible with NATO systems and a threat to F-35 fighter jets.
The United States has urged Ankara to back off its purchase of the system and has turned up the heat as its delivery date has neared.
President Recep Tayyip Erdogan has repeatedly said his country was "determined" to proceed with the Russia purchase, which he has long labeled a “done deal.”
Turkey, a NATO ally of the United States, had already sent personnel to Russia to start training with the S-400. It also has at least two pilots training in the United States to fly the F-35 – of which Turkey has purchased 100 for its own military’s use.
Turkey has spent billions to gear up its industry to participate in parts production related to the F-35s. Companies in the country make hundreds of parts for the warplane, largely for the aircraft's landing gear and center fuselage.
Lord said Turkey's participation would be finished by early 2020 if it does not reverse the S-400 deal.
"Turkey still has the option to change course," she said. "If Turkey does not accept delivery of the S-400, we will enable Turkey to return to normal."
The F-35s are designed to operate in sync with NATO systems, including antimissile defenses, raising fears that Russia could adjust the S-400's capacities against the Western alliance through information gathered in dealing with Turkey.
U.S. officials have pressed Turkey to switch to the American-made Patriot air-defense system, telling Ankara that would allow it to continue participation in the F-35 program.
Erdogan said he told the United States that Ankara would consider buying Patriots only if Washington's conditions for a deal were as good as Moscow's offer.
"But unfortunately we haven't received a positive proposal from the American side on the subject of Patriots like the S-400s from Russia," he said.
Shanahan said he believed the U.S. offer for the Patriots was "very competitive."