Four US senators have introduced a bill that seeks to block the transfer of F-35 Joint Strike Fighter aircraft to Turkey until Ankara gives up on the deal to procure S-400 air defence system from Russia.
Turkey is a key partner in the multinational Joint Strike Fighter programme, supplying parts for the stealth aircraft.
The bill was introduced by Jeanne Shaheen, James Lankford, Thom Tillis, and Chris Van Hollen. They have expressed concerns that the purchase of S-400 would endanger the integrity of the Nato alliance and compromise US defence systems.
The US and its Nato allies fear that the procurement of the Russia defence systems by Ankara will affect the security and capabilities of the F-35 aircraft.
Washington has been stepping up pressure on the Tayyip Erdogan-led government to annul the deal for the purchase of S-400 with Russia.
Pressure comes amid fears that the radar on the Russian missile system would learn how to spot and track the F-35.
Turkey is expected to receive the F-35 towards the end of 2020, while the first S-400 delivery is scheduled for July.
Shaheen said: “Make no mistake, the Kremlin is an adversary of the US and many of our Nato allies. The prospect of Russia having access to US aircraft and technology in a Nato country, Turkey, is a serious national and global security risk.
“Turkey is a critical ally, but until President Erdogan forgoes his perplexing efforts to acquire the S-400 air defence system, not a single F-35 aircraft should be delivered to Turkey.
“This bill makes it clear that Nato’s integrity, interoperability and security is a top foreign policy concern across all branches of the US Government.”
The US also offered the Patriot anti-missile system as an alternative.
However, Turkey has refused to compromise, saying that the deal cannot be scrapped.
Turkish Foreign Minister Mevlut Cavusoglu was quoted by Reuters as saying: “We have agreed with Russia in the end and signed an agreement. This agreement is valid (and) we are discussing when the delivery will be. This agreement is a done deal.”
The US is also contemplating the removal of Turkey from the F-35 programme.