The United States has stopped shipping equipment related to the F-35 fighter jet to Turkey. Washington’s move comes after Ankara insisted on purchasing the Russian S-400 missile defense system.

Turkey has only received one unit of the fifth-generation fighter jet thus far. But all current and future shipments of equipment related to the aircraft have been halted, Pentagon said, unless Ankara gives up its commitment to take delivery of a Russian S-400 missile defense system.

Also on rt.com US senators draft bill to stop transfer of F-35 jets to Turkey unless it ditches S-400 deal

A NATO ally of the US, Turkey was planning on ordering 100 F-35s, and had signed on to produce fuselage and other components for the next-generation aircraft. However, US lawmakers moved to exclude Turkey from the program, fearing Turkey’s planned purchase of the Russian-made S-400 air defense system could give the Russian system opportunity to learn how to track and spot the jet.

US President Donald Trump signed a bill last year blocking the transfer of F-35 fighter jets to Turkey, but the F-35 Joint Program Office said it would execute the transfer according to existing plans. Four US Senators introduced another bipartisan bill last week to prohibit the transfer until Ankara agrees to abandon the S-400 deal.

Also on rt.com ‘We aren’t slaves’: Erdogan defies US pressure over S-400 deal with Russia, says S-500 may follow

At present, the deal remains on the table. "The contract with Russia on S-400s remains in force and these defensive systems will be delivered to Turkey. Now talks on this issue are underway," Turkish Foreign Minister Mevlut Cavusoglu stated on Friday, according to Russian news agency TASS.

The first of four S-400 batteries will be delivered to Turkey this July. Washington has thus far failed to woo its NATO ally away from the Russian system and to Raytheon’s more expensive Patriot system.

Turkey has been a vital partner in the F-35 program, which itself has been plagued by cost overruns, delays and performance issues while being the US military's most expensive project to date. Turkey has been making parts of fuselage, landing gear and cockpit displays for the jets and has invested some $1.25 billion in the trillion-dollar program. Despite Turkey’s integral role, US officials told Reuters last week that Washington can press ahead without the Turkish components, although doing so will drive the cost of the project up further.

The first F-35 jet was delivered to Turkey in a ceremony in Texas in June. Throughout the dispute, Turkish pilots have been training on the F-35 at an airbase in Arizona since last July.

to RT newsletter to get stories the mainstream media won’t tell you.