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 U.S. 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.