Relations between Washington and Ankara have entered a downward spiral over the latter's plans to acquire Russian S-400 air defence systems. The US has warned Turkey that it could suspend the delivery of F-35 jets to the country over fears that S-400 radars could expose the planes’ weaknesses, despite Turkey producing some of the fighter's parts.

US senators have introduced a bipartisan bill to stop transfer of F-35 fighter jets to Turkey amid a major rift between the NATO allies over Ankara's purchase of Russia's S-400 missile defence systems.

One of the four co-authors of the bill, Democratic Senator Jeanne Shaheen claimed that "the prospect of Russia having access to U.S. aircraft and technology in a NATO country, Turkey, is a serious national and global security risk."

This comes shortly after Reuters reported citing anonymous sources that the US is mulling not only denying the delivery of F-35s to Ankara amid tensions over Russia's S-400 deal with Ankara, but also cutting Ankara out of the jet's supply chain.

READ MORE: Turkish Defence Minister DISCLOSES F-35 Delivery Date as Pentagon Urges Freeze

At present, Turkish factories are producing around 800 parts for the fifth-generation jet — with some of them being made only by Turkey. Despite this fact, the US believes that Turkey can be replaced by companies located in other countries.

"Turkey is not too big to fail", one of the sources said.

Finding substitutes for the Turkey's production of parts for the F-35will take time and slow down the delivery of the aircraft "for a three-month period", the source added. Neither the Pentagon, nor Lockheed Martin, which assembles the jets, has issued any official comments on the report.

Earlier, Katie Wheelbarger, the acting assistant secretary of defence for international security affairs, told Reuters that Washington had begun preparations to freeze shipments that were supposed to precede the F-35 deliveries to Turkey in a bid to "send signals" to Ankara. Wheelbarger also hinted that the US might find an alternative to Turkey's "engine depot", allegedly referring to an F-35 engine overhaul depot in Eskisehir.

Apart from engine parts, Turkey produces fuselages, cockpit displays, and landing gear for the F-35 jets. This matter has become a stumbling block for the US in its plans to halt the deliveries of the fifth-generation jets to Turkey following the spat over Ankara's S-400 procurement.

Washington fears that the Russian air defence systems' radars could learn to detect the top stealth fighter if Turkey obtains both S-400s and F-35 jets. In light of these concerns, the US has repeatedly called on Turkey to drop the S-400 deal and to buy the more expensive Patriot systems, which are only capable of downing ballistic missiles, as opposed to the Russian S-400, which is capable of downing enemy jets, drones, and missiles.

READ MORE: US to Hand Over Two Additional F-35s to Turkey Amid S-400 Spat — Report

Turkey has slammed the US pressure and repeatedly reiterated its intention to stick to the S-400 agreement and demanded the timely delivery of F-35 jets from Washington, noting that it has both invested in their development and paid for their acquisition.