The Pentagon has warned Turkey that it will suspend training of its pilots on the advanced F-35 fighter jet at the end of July and terminate Turkey’s participation in the fighter jet programme if it buys Russia’s S-400 missile defence system. Acting Defence Secretary Patrick Shanahan, in a letter sent on Thursday to Turkey’s Minister of Defence Hulusi Akar, said Turkish pilots at Luke Air Force Base in Arizona would have time to complete their flight training by July 31, but no new students would be accepted. In addition, Turkey’s participation in the programme, such as producing hundreds of parts that make up the F-35s fuselage and engines, would be phased out. The Pentagon said the phase-out would occur over time so as not to disrupt the F-35 production line, but would end early next year. “All actions taken on the F-35 are based on risks the S-400 presence in Turkey would have,” Shanahan wrote to his Turkish counterpart. Four F-35’s Turkey bought are still in US custody at Luke AFB and will not be transferred to Turkey at this time, a defence official said. The Pentagon is considering whether Turkey could be reimbursed for the jets, which now cost roughly US$90 million each. “We are under discussions internally as to how to deal with the four aircraft they have already taken delivery on,” said Ellen Lord, under secretary of defence for acquisition and sustainment. There are 42 Turkish pilots taking part in F-35 flight training at Luke, the Pentagon said. Thirty-four of them may be able to complete their training by the July deadline, but if not, their access will be cut off. “All Turkish personnel, including two instructor pilots, in the US related to the F-35 programme will be required to depart the country July 31, 2019. At this point, all Invitational Travel Orders (ITOs) and/or Common Access Cards (CACs) will be cancelled, and Turkish Air Force personnel will be prohibited from entering Luke AFB or Eglin AFB and applicable buildings,” the Pentagon said. The US has warned Turkey repeatedly that it would not allow the transfer of the advanced fighter jet, which is produced by Fort Worth, Texas-based Lockheed Martin Aeronautics, to Turkey if it bought Russia’s missile defence system. US officials worry that if Turkey uses the Russian system in conjunction with the F-35, it could result in some of the fighter jet’s classified capabilities or vulnerabilities being revealed to potential US adversaries. “We do not want to have the F-35 in close proximity to the S-400 for a length of time, because of the ability to understand the profile of the F-35,” Lord said. The Pentagon also said the suspension of sending any F-35 support equipment to Turkey would continue and the US would look at alternative sources to supply the components Turkey has been producing. Lord said Turkey provides 937 parts for the jet. The Pentagon is now working with Lockheed to find replacement sources for the parts and Pratt & Whitney for substitute supply lines for the engines. The US said it will continue to conduct military training exercises with Turkey. The extended time frame for when the Turkish pilots would have to leave the United States was intended to leave the door open for the Turkish government to cancel its order from Russia. However it was unclear whether US F-35 jets would join exercises with Turkey if it completes its acquisition of the Russian missile defence system.