The Pentagon announced on Friday that if Turkey did not give up on the S-400 system by July 3, Ankara would be blocked from purchasing F-35 fighter jets and Turkish pilots currently training in the US would be expelled.
US acting Secretary of Defense Patrick Shanahan said last week he had sent a letter to his Turkish counterpart Hulusi Akar informing him of Washington's decision to pull Turkey out of the F-35 programme.
"Turkey has already bought S-400 defense systems. It is a done deal. I hope these systems will be delivered to our country next month," Turkish President Recep Tayyip Erdogan said on Wednesday.