The Pentagon is getting close to slamming the door on Turkey regarding the contentious transfer of Lockheed F-35 stealth jets purchased previously by Ankara, an issue debated this week in Congressional hearings. The Department of Defense (DoD) has now offered a stern ultimatum and warning, telling Turkey: don't expect to receive F-35s if Russia's anti-air defense system is bought.

F-35B Stealth fighter jet, file photo

Pentagon spokesman Charles Summers said Friday morning that there will be "grave consequences" if Turkey moves forward in purchasing Russia's S-400, according to Bloomberg — this after the top US commander in Europe, Army General Curtis Scaparrotti, recommended to Congress this week that delivery to Turkey of Lockheed Martin's F-35 Joint Strike Fighter should ultimately be cancelled, nothing that the S-400 remains “a problem to all of our aircraft, but specifically the F-35.”

Turkey's currency fell 1.5 percent this week amid fears relations with Washington could worsen over the standoff. President Erdogan has further missed a "soft deadline" previously set by the US over an offer to buy $3.5 billion Raytheon Co. Patriot missile shield system, as an alternative to the Russian S-400. The Pentagon said the Patriot offer would be impossible should Turkey seek the Russian system.

Failure to receive the F-35s could further impact Turkey's economy given that a number of Turkish defense technology companies have contracts to build and develop systems and add-ons needed for Turkey to operate the aircraft, such as cockpit displays for the multi-national fighter jet.

However, Erdogan has remained unmoved as the issue has come to a head, repeating during a Turkish TV broadcast interview this week, "this is over" in reference to continued debate. “There can never be a turning back. This would not be ethical, it would be immoral. Nobody should ask us to lick up what we spat,” he said.

Thus far Erdogan has dismissed all Pentagon and US ultimatums. "We are an independent Turkey, we are not slaves," he said during the interview previously this week.

The advanced Russian-made S-400 air defense system sought by Turkey has been seen as a threat by the United States, given the potential for compromising the F-35 advanced radar evading and electronics capabilities.

The main argument for blocking the F-35 transfer is the fear that Russia would get access to the extremely advanced Joint Strike Fighter stealth aircraft, enabling Moscow to detect and exploit its vulnerabilities. Russia would ultimately learn how the S-400 could take out an F-35.

Also this week the US State Department warned that transferal of S-400 systems could result in far reaching sanctions against Turkey under the Countering America's Adversaries Through Sanctions Act (CAATSA), despite its NATO status.