US lawmakers have been attempting to thwart Turkey's purchase of Russian S-400 air defence systems ever since the deal was signed in December 2017. Some senators suggested suspending F-35 shipments to Ankara as a measure to dissuade Turkey. This reported by Sputnik.

US General Curtis Scaparrotti told senators in the Senate Armed Services Committee that he doesn't recommend delivering F-35 jets to Ankara if it doesn't abort the S-400 missile defence deal with Russia. Scaparrotti is the current head of the United States European Command.

The general noted that he, as a military officer, doesn't advise allowing the top US jets to be used in countries that operate Russian military systems, especially air defence systems.

"If they accept the S-400 and establish it within Turkey, there is an issue […] that has to do with the F-35. It presents a problem to all of our aircraft, but specifically the F-35. My best military advice would be that we don't follow through with the F-35[delivery], flying it or working with an ally that's working with Russian systems, particularly air defence systems", he said.

The other issue that Scaparrotti raised was the fact that Russian air defence systems are not interoperable with the existing integrated air and missile defence systems set up by NATO allies.

At the same time, Scaparrotti noted that Turkey remains an important strategic ally for the US, with 334 open foreign military sales (FMS) cases worth over $9.9 billion in total.

"These sales are important components of the broad US-NATO effort to ensure interoperability of equipment and combined training in Western tactics, techniques and procedures", the general said.