NEW YORK    -   The US Pentagon has notified Turkey that it is cancelling its purchase of F-35 fighter jets if the Turkish government goes ahead with the purchase of a Russia’s S-400 missile defence system.

Acting US Defense Secretary Patrick Shanahan has sent a letter notifying Turkey that all training of Turkish pilots will also end as of July 31. And all Turkish personnel connected to the F-35 programme must leave the country by the end of that month.

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 programme; 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.”

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 two NATO allies have sparred publicly for months over Turkey’s order for Russia’s S-400 air defence system, which Washington says poses a threat to the Lockheed Martin Corp F-35 stealthy fighters, which Turkey also plans to buy.

The United States has said Turkey cannot have both, but had avoided taking steps to curtail or halt planned training of Turkish pilots in the programme, a reprisal that could be seen as an embarrassment in Turkey.

The announcement came as the head of Russia’s state-owned conglomerate Rostec said Moscow would begin delivering the S-400 air defence systems to Turkey. “Everything is on track with the Turks. I hope that we will begin to deliver in about two months,” Sergei Chemezov told NTV channel, according to Russian news agencies on Friday.

“The credit money has been spent, the technology was produced. And we completed training of all the military personnel,” he said. If Turkey were removed from the F-35 programme, it would be one of the most significant ruptures in recent history in the relationship between the two allies, experts said. Turkish officials have not immediately commented on the Pentagon’s letter. Turkish President Recept Tayyip Erdogan said on Tuesday it was “out of the question” for Turkey to back away from its deal with Moscow. Erdogan said the US had not “given us an offer as good as the S-400s”.

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

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

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

The Pentagon believes that it can minimise the effects on the broader programme 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 undersecretary of defence, told reporters at the Pentagon. But if Turkey were removed from the F-35 programme, the ramifications would be felt far beyond the Turkish air force.

Strains in ties between the US 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.