The United States on Friday raised the stakes in its standoff with Turkey over Ankara’s deal to acquire a Russian air defense system, laying out a plan to remove the NATO ally from the F-35 fighter jet program that includes immediately halting any new training for Turkish pilots on the advanced aircraft.

Acting US Defense Secretary Patrick Shanahan sent a letter to his Turkish counterpart, seen by Reuters on Friday, that laid out the steps to remove Turkey from the program unless Turkey changes course.

Reuters on Thursday first reported the decision to stop accepting more Turkish pilots for training in the United States, in one of the most concrete signs that the dispute between Washington and Ankara is reaching a breaking point.

The United States says Turkey’s acquisition of Russia’s S-400 air defense system poses a threat to the Lockheed Martin Corp F-35 stealthy fighters, which Turkey also plans to buy. The United States says Turkey cannot have both.

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Shanahan’s letter explicitly states there will be “no new F-35 training.” It says there were 34 students scheduled for F-35 training later this year.

“This training will not occur because we are suspending Turkey from the F-35 program; there are no longer requirements to gain proficiencies on the systems,” according to an attachment to the letter that is titled, “Unwinding Turkey’s Participation in the F-35 Program.”

Turkish personnel already in the United States will see their training on the F-35 discontinued at the end of July.

In his letter, Shanahan also warned Ankara that its deal with Moscow risked undermining its ties to NATO, hurting the Turkish economy and creating over-dependence on Russia.

“You still have the option to change course on the S-400,” Shanahan wrote.

The Turkish lira declined as much as 1.5% on Friday before recovering some losses. The currency has shed nearly 10% of its value against the dollar this year in part on fraying diplomatic ties and the risk of U.S. sanctions if Turkey accepts delivery of the S-400s.

Graceful exit?

Turkey is one of the core partners in the F-35 program and expressed an interest in buying 100 of the fighters, which would have a total value of $9 billion at current prices.

Turkish companies produce some 937 parts of the F-35, largely for the aircraft’s landing gear and center fuselage, the Pentagon says. The United States is now planning to move that production elsewhere, ending Turkey’s manufacturing role by early next year.

The Pentagon believes that it can minimize the impact on the broader program if Turkey abides by the US timeline.

“What we are doing is working to do a very disciplined and graceful wind down,” Ellen Lord, an under secretary of defense, told reporters at the Pentagon.

But if Turkey were removed from the F-35 program, the ramifications would be felt far beyond the Turkish air force. It would be one of the most significant ruptures in recent history in the relationship with the United States, experts said.

Strains in ties between the United States and Ankara already extend beyond the F-35 to include conflicting strategy in Syria, Iran sanctions and the detention of US consular staff in Turkey.

Still, Turkey seems to be moving ahead with the S-400 purchase, regardless of the US warnings. President Recep Tayyip Erdogan said on Tuesday it was “out of the question” for Turkey to back away from its deal with Moscow.

Defense Minister Hulusi Akar said on May 22 that Turkish military personnel were receiving training in Russia to use the S-400, and that Russian personnel may go to Turkey.

The head of Russian state conglomerate Rostec, Sergei Chemezov, was quoted as saying on Friday that the country would start delivering S-400 missile systems to Turkey in two months.

Erdogan said the United States had not “given us an offer as good as the S-400s.”

Still, Lord held out hope for Turkey, noting that none of the US decisions so far were irreversible.

“We’re hopeful that they will stop the acquisition of the S-400. We would very much like them to stay in the (F-35) program,” Lord said.