The United States said it was halting deliveries to Turkey related to the F-35 stealth fighter jet program and warned that Turkish participation in the manufacture of the aircraft was at risk after the NATO ally purchased Russia’s S-400 air and missile defense system.

“The United States has been clear that Turkey’s acquisition of the S-400 is unacceptable,” Acting Chief Pentagon spokesperson Charles E. Summers Jr. said in a statement on Monday, April 1.

“Our important dialogue on this matter will continue, however, until they forgo delivery of the S-400, the United States has suspended deliveries and activities associated with the stand-up of Turkey’s F-35 operational capability,” Summers said.

But U.S. Acting Defense Secretary Patrick Shanahan said on Tuesday he was confident the situation would be resolved.

Turkey has planned to acquire 100 F-35As. The first two Turkish F-35 jets were delivered in June 2018 and are based at Luke Air Force Base in Arizona, where Turkish pilots and maintainers train with the aircraft. Two further aircraft are expected to be delivered to Luke in November, Turkey’s state-run Anadolu Agency reported. Defense Minister Hulusi Akar also said last month that he expected delivery of F-35s to an airbase in eastern Turkey in November.

The sale to Turkey of Russian S-400 air and missile defense systems has long been contentious.

Concerns have been raised by Turkey’s NATO allies about data security with the S-400 system, particularly around the F-35. Turkey’s Undersecretariat for Defence Industries (SSM) wants to connect the F-35 systems with the Turkish Air Force information network, HvBS. If the S-400 is also connected to the HvBS, there is a risk that data collected by the advanced Joint Strike Fighter’s sensors may end up being transmitted to Russia.

There are also fears that F-35s flown in Turkey could be detected by its own S-400 radar systems, and that data sent to Russia could be used to improve detection and targeting of the stealthy F-35 by Russian equipment.

During a U.S. Senate Armed Services Committee hearing last month, U.S. Army General Curtis Scaparrotti, European Command commander and NATO Supreme Allied Commander Europe, said that he would recommend that the U.S. should not sell the F-35 to Turkey if it acquired the S-400.

“My best military advice would be that we don’t then follow through with the F-35, flying it or working with an ally that’s working with Russian systems, particularly air defense systems,” Scaparrotti said.

Update April 2 President Donald Trump’s nominee to replace Scaparotti as head of NATO and EUCOM echoed those remarks at his Senate Armed Services Committee confirmation hearing on Tuesday.

“I concur with this committee’s belief that the S-400 and the F-35 are not compatible, and if Turkey proceeds down a path to procure and operate the S-400, they should not get the F-35,” U.S. Air Force Lieutenant General Tod Wolters said, adding that “it’s absolutely unsustainable to support co-location of an F-35 and S-400.”

After much discussion, Turkey signed a deal to purchase the S-400 system in September 2017 when an advance payment was made.

Just last week, Foreign Minister Mevlut Cavusoglu met his Russian counterpart Sergei Lavrov and reiterated that the S-400 purchase would go ahead, saying that Turkey has “an agreement with Russia and we are bound by it.” Turkey’s Defence Industries undersecretary previously said he expects the first S-400 delivery in July.

The suspension announcement comes just two days before foreign ministers from all 29 members of NATO are due to meet in Washington to celebrate the 70th anniversary of the alliance, and the day after President Recep Tayyip Erdogan’s AK Party suffered heavy losses in municipal elections across Turkey.

Pentagon threatens to remove Turkey from F-35 manufacture program

But companies based in Turkey manufacture critical parts as part of the F-35 Joint Strike Fighter program, and Summers also warned on Monday that Turkey may be removed entirely from the F-35 program, saying that the Department of Defense has “initiated steps necessary to ensure prudent program planning and resiliency of the F-35 supply chain.”

“Should Turkey procure the S-400, their continued participation in the F-35 program is at risk,” Summers said.

“Secondary sources of supply for Turkish-produced parts are now in development,” he added.

In July 2018, then-Secretary of Defense James Mattis warned of significant delays if Turkish companies were removed from the F-35 program.

“If the Turkish supply chain was disrupted today, it would result in an aircraft production break, delaying delivery of 50-75 jets and would take approximately 18-24 months to re-source parts,” Mattis wrote in a letter to the House Armed Services Committee chairman, Bloomberg reported.

Turkey-based companies manufacture parts for all F-35 variants and customers. The plane’s manufacturer, Lockheed Martin, said that contracts with Turkish companies to build F-35 parts had been expected to reach $12 billion.

According to Forecast International, Turkish Aerospace Industries manufactures center fuselages as a second-source supplier and is slated to deliver 400 in total. TAI also produces weapons bay doors, air-to-ground pylons, air inlet ducts and other components.

Other Turkish suppliers include: Ayesas, which is the sole source supplier of both the panoramic cockpit display and the missile remote interface unit; Alp Aviation, which makes metal airframe structures, landing gear components, and F135 engine parts; and Fokker Elmo Turkey (GKN), which makes the Electrical Wiring and Interconnection Systems (EWIS) for the F-35 and the F135 engine.

Turkey was also selected as a Regional F135 Maintenance, Repair, Overhaul, and Upgrade Facility.

Potential Turkish air defense missile system purchases

Despite neighboring conflict zones in the Middle East, Turkey doesn’t have a sophisticated long-range air defense system. Over the past five years, NATO has deployed up to six Patriot missile defense batteries to protect Turkey’s southern border against missiles fired from Syria.

Update, April 2: On Tuesday, U.S. Acting Defense Secretary Patrick Shanahan said he was confident that Turkey would drop its plan to purchase the S-400 and buy the U.S. Patriot system instead.

That would then allow the F-35 program to continue, he said.

“I’ve had a number of conversations with Defense Minister Akar and I really think we’ll resolve this situation with our strategic partners,” he said.

“I am very confident in the Patriot proposal that we’ve delivered to Turkey, its availability, it’s pricing, and very importantly, the industrial participation that comes along with the Patriot system,” adding that he expects Turkey will buy the Raytheon-made Patriot system.

According to Russian press reports last April, Turkey was interested in a technology transfer and even joint production of the S-400 with Russia, a prospect that was resisted by some in the Moscow security services. Foreign Minister Mevlut Cavusoglu said in October 2017 that Turkey could annul the S-400 deal if Moscow refuses to transfer technology.

But Turkey is not going to rely solely on the S-400 for air defense.

The U.S. Department of State cleared a possible $3.5 billion sale of Patriot air defense systems to Turkey in December, but the previous month, although the Department of Defense had warned that that acquisition, along with those of CH-47F Chinook heavy lift helicopters, UH-60 Black Hawk utility helicopters, and F-16 Fighting Falcon aircraft could be affected by its going ahead with the S-400 purchase.

Then-Defense Minister Nurettin Canikli said in November 2017 that Turkey agreed with Eurosam to develop an air defense missile system and to own the technology with Turkey’s “own local resources.” Canikli signed a letter of intent with the defense ministers of France and Italy that month, paving the way for Turkey’s purchase of Eurosam SAMP-T launcher systems and Aster 30 surface-to-air missiles.

Turkey is also developing its own HISAR short-range, low-altitude air defense missile system intended to target aircraft, helicopters, cruise missiles and unmanned aerial vehicles.

With reporting from AFP