Us threatens turkey with sanctions in case of S-400 purchase reports

The United States has urged Turkey to abandon the purchase of the S-400 anti-aircraft missile system from Russia, warning Ankara of the consequences of its decision, Haberturk reported citing a source in Washington.
According to*the Turkish newspaper, an unnamed American official expressed his concerns over*Ankara’s purchase of*Russian surface-to-air missile systems, explaining that they could “negatively influence the interoperability of*NATO” and assuming that Washington could impose sanctions in*response to*the recently ad“We want to*help Turkey find a better alternative to*meet its air defense needs,” the official said.opted law.
Earlier in*February, Turkish Foreign Minister Mevlut Cavusoglu spoke out*against the language of*threats regarding its purchase of*the S-400 systems.
"We do not use and will not accept the language of*threats. Yes, the US Congress adopted a law [on anti-Russia sanctions]. But we need to*ensure our national security. We would like*to purchase the means to*ensure it from*our allies … We spoke to*not only Russia but*with other countries too. We finished talks with*Russia long before*the Congress, only small details were left," he told a press conference with*his American counterpart Rex Tillerson.
The US State Department has previously threatened to*apply “appropriate measures” against*countries willing to*buy Russia’s S-400 systems.
In December 2017, Moscow and Ankara finalized an agreement on*the delivery of*the S-400 surface-to-air missile systems.

Russia must be rather desperate for exports. Selling some of their best hardware to a NATO nation seems like a bad idea when all the other members will learn how to counter it.
Arguably, the S-400 SAM system is the biggest threat to NATO airpower in the Middle East & Eastern Europe.

On the other hand when Turkey tries to harmonize it within the NATO framework Russia will get an all access pass into how it picks up NATO aircraft and especially with the F-35/22. Not to mention when Russia sells this sort of kit it usually sends along a team to work it for the country

The US is very much aware of the issue regarding the F-35 & it seems like there's pressure on Congress (unsurprisingly including some from Greece) to cancel Turkey's F-35 order.
There's also suggestions that the Lockheed Martin's Autonomic Logistics Information System may essentially provide a kill switch on the F-35 which could prevent use of the aircraft even after it's been purchased.
At the very least, it appears very unlikely that NATO are prepared to take precautions to ensure stealth is not compromised.

http://www.thedrive.com/the-war-zone/17687/turkey-wants-to-link-its-f-35-computer-brains-to-networks-that-will-include-russian-systems

https://ahvalnews.com/us-turkey/us-caution-grows-over-turkish-f-35s-lobbyist