Patrick Shanahan

Acting Defense Secretary Patrick Shanahan stepped down on June 18 before his formal nomination ever went to the Senate, citing a "painful" family situation that would hurt his children and reopen "wounds we have worked years to heal."

U.S. Acting Defense Secretary Patrick Shanahan and his Turkish counterpart Hulusi Akar remain in contact about Ankara's plans to buy a Russian air defense system, and may meet during NATO meetings in Brussels next week, NATO's commander told Reuters.

Turkish President Recep Tayyip Erdogan said he expected Russian S-400 missile defense systems to start arriving in Turkey in the first half of July, broadcaster NTV reported on Sunday, a development set to fuel tensions with NATO ally Washington.

Turkey Defense Minister Hulusi Akar had a phone call with U. S. Acting Secretary of Defense Patrick Shanahan, following the latter's letter announcing Washington's decision to cease Turkish participation in the F-35 aircraft project.

Turkey will not back down from its decision to buy Russian S-400 missile defense systems and rejects any ultimatums on the issue, Foreign Minister Mevlut Cavusoglu said on Thursday in response to US warnings about the purchase.

The language used in a letter sent from Washington to Ankara regarding Turkey's removal from the F-35 fighter jet program does not suit the spirit of alliance, Turkish Defense Minister Hulusi Akar said on Wednesday.

U.S. Acting Defense Secretary Patrick Shanahan said on June 11 he will discuss Washington's spat with Ankara over its purchase of Russia's S-400 missile defense system with his Turkish counterpart Hulusi Akar on June 12.   

Comments last week by Acting US Defense Secretary Patrick Shanahan, who drew parallels between Moscow's "unsafe and unprofessional" behavior following the near-collision between US and Russian warships in the Philippine Sea and Turkey's moves in the Eastern Mediterranean, pointed to something much more profound and significant than the usual occasional statements.

The United States has not moved to create a joint working group to assess its concerns regarding Turkey's purchase of the Russian S-400 missile defense systems, the head of the Turkish Defense Industries Directorate said on June 10.

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.