In the latest in the continuing fallout between the US and Turkey over Ankara's plans to install Russia's S-400 missile defense systems — for which the United States this week finally halted delivery of not only the jets but also equipment related to the stealth F-35 fighter aircraft, and canceled all future shipments of F-35 related material — Turkey has issued its own counter-ultimatum: you are either with us or with the terrorists.

In response to yesterday's threat by Vice President Mike Pence putting Turkey on notice to either scrap the S-400 deal with Russia or say goodbye to the Lockheed F-35, Turkish Vice President Fuat Oktay tweeted, “The United States must choose. Does it want to remain Turkey’s ally or risk our friendship by joining forces with terrorists to undermine its NATO ally’s defense against its enemies?”

File photo via RFE/RL

Ironically it's precisely the "undermining of NATO defense" that has Washington concerned related to the S-400, given the potential for compromising the F-35 advanced radar evading and electronics capabilities as Russia could get access to the extremely advanced Joint Strike Fighter stealth aircraft, enabling Moscow to detect and exploit its vulnerabilities.

Pence said during a speech Wednesday at the "NATO Engages" summit in Washington, DC: "We've also made it clear that we'll not stand idly by while NATO allies purchase weapons from our adversaries, weapons that threaten the very cohesion of this alliance."

"Turkey's purchase of a $2.5 billion S-400 anti-aircraft missile system from Russia poses great danger to NATO and to the strength of this alliance," he added, warning further that Turkey could face serious consequences.

While Turkey's VP didn't specify which "terrorists" the US would be taking sides with as part of his response, there's little doubt this was a reference to Syrian Kurdish militias which has also been the source of tensions given ongoing US support to the Kurdish-led Syrian Democratic Forces (SDF) just across Turkey's border with Syria. Over the past year Washington has repeatedly warned against Turkish forces foraying deeper into Syria to fight Syrian Kurdish militants.

The United States must choose. Does it want to remain Turkey’s ally or risk our friendship by joining forces with terrorists to undermine its NATO ally’s defense against its enemies? — Fuat Oktay (@fuatoktay06) April 3, 2019

Turkey has also in the past lashed out at Washington over the fact that self-exiled Turkish cleric Fethullah Gulen has been allowed safe haven in the US. Turkey has long claimed he was a prime force behind the 2016 coup attempt targeting the Erdogan government.

On Wednesday Turkey sought to calm US fears of compromising NATO's systems, as Foreign Minister Mevlut Cavusoglu stated, “[S-400s] will not be integrated into the NATO system,” and further suggested establishing a multi-party technical group that would ensure the air defense system “will not be a threat” to any advanced western systems, the F-35 included.

US Vice President Mike Pence warns Ankara against buying a Russian S-400 anti-missile system, saying Turkey must choose between remaining a critical partner in NATO or making what he called “reckless decisions” pic.twitter.com/TnqisikaPY — TRT World Now (@TRTWorldNow) April 3, 2019

Both the US and Turkish Vice Presidents publicly sparring this week will likely only harden Erdogan in his position. Turkey again reminded the world last Friday that, "We have signed a deal with Russia, and this deal is valid. Now we are discussing the delivery process," according to words from the foreign ministry on Friday after he came out of a meeting with his Russian counterpart FM Lavrov.

He added that "We have an agreement with Russia and we are bound by it." The first Russian S-400 delivery is expected in July.

Both last week President Erdogan and Turkish officials have remained unwavering in declaring "it's a done deal" in the face of US threats.

A month ago Erdogan even colorfully told a Turkish broadcaster during an interview that, “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.”