Washington has cautioned that the S-400 system might covertly obtain critical information on the advanced fighter jets, including their detection range, which could then be relayed to Russia.
Should Turkey decide to purchase a Russian missile defence system, it would risk being unable to access a wide gamut of US-supplied weapons systems, including the F-35, the top US general for Europe said Tuesday.
General Curtis Scaparrotti told the Senate Armed Services Committee that if Turkey takes delivery of the Russia-made S-400 missile defence system it would "potentially forfeit many of the other systems, and one of the most important systems that we provide them," alluding to the F-35.
"I would hope that they reconsider this one decision on S-400," Scaparrotti, who also serves as NATO's supreme allied commander, said.
Scaparrotti pointed to several problems the Russian system poses, including a lack of interoperability with NATO systems, and said the S-400 is "a problem to all of our aircraft, but specifically the F-35, I believe.
"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 is working with Russian systems, particularly air defence systems, with one of our most advanced technological capabilities," he said.
US State Department spokesman Robert Palladino also said that, "We’ve long made it clear we would like to work collaboratively on air and missile defence with Turkey, and we have offered opportunities for Turkey to consider Patriot among other systems over the years. We’ve also made it clear to Turkey that we have very serious concerns with its stated plans to proceed with the acquisition of the Russian S-400 missile defense system."
He added that Turkey's acquisition of the Russian air defence system "will result in a reassessment of Turkey's participation in the F-35 program and risk other potential future arms transfers to Turkey, as well as lead to potential sanctions under Countering America's Adversaries Through Sanctions Act."
Washington has cautioned the S-400 system might covertly obtain critical information on the advanced fighter jets, including their detection range, which could then be relayed to Russia.
After protracted efforts to purchase air defence systems from the US with no success, Ankara decided in 2017 to buy Russian S-400 air defence systems.
Turkey's Foreign Minister Mevlut Cavusoglu said last week Ankara and Washington have begun negotiating the potential purchase of Patriot missile defence systems from the US.
The State Department approved in December a possible $3.5 billion sale of Patriot systems to Turkey that includes advanced radar systems, control centres, launching systems and guided missiles.
Ankara is planning to purchase 100 F-35 fighter jets from the US pending congressional approval, and its pilots are currently training on the weapons platform at Luke Air Force Base in Arizona.
Turkish firms also supply the F-35 program with key components, including airframe structures and assemblies, and the centre fuselages.
Asked by Sen. Jeanne Shaheen who "picks up that slack if Turkey can't receive the F-35," Scaparrotti said the issue is currently being considered.
"This is a huge decision for Turkey," he said. "It connects in many different ways to the employment and the integration that they have within the system itself."
Why is Turkey buying S-400s?
First, an air defence system is perceived as a 'must-have' for Turkey. S-400s can defend a large area and can hit targets as far as 250 miles away which can counterbalance threats from the region.
Secondly, Turkey is pursuing a more independent foreign policy through the purchase that aims to protect its airspace and counter threats beyond its borders as conflict rages on in neighbouring Iraq and Syria.
An added bonus for Turkey is that the S-400 is cheaper than other defence systems in the same league.
In 2013, Turkey launched a tender aiming to purchase defence systems. A Chinese company, (CPMIEC), suggested the most affordable price, but America forced Turkey to drop the deal by saying that the company had been sanctioned by the US for alleged missile sales to Iran.
What exactly is the S-400?
The S-400 Triumf is Russia's next-generation, most advanced long-range anti-aircraft missile system.
It is designed to detect, track and then destroy any machine that poses a threat.
The Triumf is twice as effective as other anti-aircraft missile systems. It can work with 4 different types of missiles with different starting weight and launch range.
Russia has also agreed to sell them to China and India.