Turkey's Defense Minister Hulusi Akar dismissed the Pentagon's latest warning over Ankara's imminent procurement of Russian missiles on Thursday as "language unbecoming of an ally."
U.S. acting Secretary of Defense Patrick Shanahan's letter to his Turkish counterpart laid out a detailed timetable of Turkey's exclusion from the purchase and production of America's latest F-35 fighter jet if Ankara buys Russia's S-400 missiles system.
The Pentagon claims the S-400 would compromise the F-35 and, in particular, its stealth technology that was developed to evade systems like the S-400.
Ankara disputes Washington's fears. Akar said he would speak with Shanahan on Thursday, and said a written response would follow in the "coming days."
The Pentagon letter is seen as a blow to Ankara. "It's a very serious escalation on the part of the United States. They've put the ball in Turkey's court," said international relations expert Soli Ozel of Istanbul's Kadir Has University.
Last month, Turkish President Recep Tayyip Erdogan claimed a diplomatic breakthrough, after a telephone call with U.S. President Donald Trump. Ankara said both sides had agreed to set up a joint committee to investigate security concerns posed by the S-400.
However, with the S-400 due to be delivered next month, time appears to be overtaking diplomatic efforts. In a further ratcheting up of bilateral tensions, the Pentagon announced the ending of training of Turkish pilots for the F-35, which it is due to receive later this year.
Analysts suggest the move took Ankara by surprise and is likely in response to local reports of the imminent arrival of Russian personnel in Turkey to prepare for the S-400 installation.
Washington's hardening of its stance comes as the S-400 controversy is threatening to isolate Ankara.
"Turkey feels more and more alone in the western world. None of the countries offer direct support for Turkey," said International relations professor Huseyin Bagci of the Middle East Technical University. "But countries like Turkey, when you put it in a corner, will never give up. It's like a cornered cat — it will attack.
"The Russians are the winner of the day," he added. "The Russians very successfully created a conflict between Turkey, NATO, and the United States."
Turkish pro-government newspaper columnists are on the attack, warning of a wave of anti-Americanism being unleashed in Turkey if Washington goes ahead with its threats. Other writers questioned whether buying the F-35 was necessary, claiming Russian jets were superior.
However, Ankara faces a great deal more pain than just the loss of F-35 jets if it proceeds with the S-400 missile deal. The U.S. Congress is warning robust financial penalties will hit Turkey's already weakened economy under Countering America's Adversaries Through Sanctions Act (CAATSA), which prohibits significant Russian defense purchases.
"Washington has to understand, it's beyond politics," said Bagci. "It's a matter of honor for Recep Tayyip Erdogan because he promised this [purchase of S-400] directly to [Russian President Vladimir] Putin, one leader to another, he cannot be seen to go back on his word.
"Turkey also cannot afford another Russian divorce; the price of the shooting down of the Russian plane was $17 billion," Bagci added.
In 2015, a Turkish fighter jet mistakenly downed a Russian warplane operating from a Syrian airbase close to Turkey's border. In response, Moscow hit Ankara with punitive trade sanctions.
'No winning solution for Turkey'
"I can understand Ankara's quandary. There is no winning solution for Turkey," Ozel said. He suggests Ankara's strategy increasingly appears to rest on the belief Erdogan and Trump can resolve the impasse themselves at a planned meeting on the sidelines of the G-20 summit in Osaka, Japan, on June 28 and 29.
"It's significant the two presidents have not commented on this at all [on the Pentagon letter], which leads everyone to speculate that the Turkish president has invested all his hopes on meeting president Trump in Osaka. The big question is, can Trump do anything to reverse this course?
"Given all the political troubles Trump has, I doubt he really wants to confront the Congress on Turkey, with both Republicans and Democrats united on this issue, especially given the Pentagon is so adamant against Turkey, and I doubt Trump will want to cross them," added Ozel.
With Moscow confirming it will deliver the S-400 by July, Bagci warns Washington needs to think carefully about its actions.
"What the Americans understand with Turkey's imperial past is it's not a country to be told what to do," Bagci said. "It's not the first time Turkey is isolated [and] when it happens, Turkey becomes insular, aggressive to its neighbors. Turkey is the cornerstone of the region; an unstable Turkey will have long-term effects on the security of the region."