e US will stop its manufacturing support for F-35 fighter jets in Turkey if Ankara continues to pursue plans to acquire Russian long-range S-400 surface-to-air missile systems,  acting Defence Secretary Patrick Shanahan told reporters on Friday.

“If Turkey decides that the S-400 is a decision they want to go forward with, then we have to move work out of Turkey”, he warned.

Shanahan pledged to discuss options with representatives of US aerospace manufacturers Lockheed Martin and United Technologies should Turkey go ahead with plans to purchase S-400s.

The statement comes as House Armed Services Committee members, including Democratic Congressman John Garamendi, announced a bill to block the sale of the F-35 warplanes to Turkey if it goes ahead with its push to buy the Russian air defences.

“Operating the S-400 alongside the F-35 would compromise the aircraft and its sensitive technology, impact interoperability among NATO allies, and most importantly pose a serious risk to our shared defence and security”, Garamendi said in a statement.

He described the bill as a document which “sends a strong and important message to Turkey — proceeding with the S-400 is unacceptable and will not be tolerated”.

This comes amid ongoing tension between the US and Turkey over Ankara inked a loan agreement for the supply of S-400 systems with Russia in December 2017.

Washington is worried that the S-400’s radars will be able to learn how to spot F-35 stealth jets, which were originally scheduled for shipment to Turkey in 2019.

The US, since then, has threatened to freeze the fighter jets’ delivery to Turkey if it doesn't drop the S-400 deal. Ankara, meanwhile, has demanded a timely delivery of the F-35s from the US, warning that the F-35 programme will be “damaged” if Washington excludes Turkey. 

Ankara, which has repeatedly signalled its commitment to the S-400 deal, insists that the S-400s are not a threat to the security of NATO, the US or the F-35 in any way. Despite this, Washington moved to stop deliveries of F-35 fighter jet parts to Turkey in early April.