Turkey accused on Tuesday the United States of violating the “spirit of the alliance” between them as it responded to Washingtons ultimatum to Ankara to abandon a deal to buy a Russian missile defense system. The Turkish defense ministry said in a statement that "the wording and approach" of a letter sent this month by outgoing Acting US Defense Secretary Patrick Shanahan "was not in the spirit of an alliance (and) caused unease". The ministry said Turkeys previously "known" opinions on acquiring the Russian S-400 missile defense system had been shared in a detailed manner, without offering further information. It stressed the importance of "continued dialogue based on mutual respect and friendship, and efforts to find an appropriate solution to the issues". Shanahans letter warned Ankara that it has until July 31 to renounce the S-400 purchase, or it would not be allowed to purchase around 100 cutting-edge F-35 fighter jets. Turkey has been associated with the project almost since the beginning, with the state-owned Turkish Aerospace making part of the planes fuselage. Turkish pilots would also be booted from training programs on the newest US combat plane, and Washington has warned that Turkey will face economic sanctions if it goes through with the deal. Ankaras push to buy the missile defense system from Moscow has caused significant tension between the NATO allies, whose ties are already strained by multiple issues including US support for a Syrian Kurdish fighters viewed as terrorists by Turkey. Despite US concerns over the S-400 systems interoperability with NATO equipment, Turkey has repeatedly said it will not back away from the deal. Turkish President Recep Tayyip Erdogan is expected to meet his US counterpart Donald Trump on the margins of a G20 summit in Japan later this month. Erdogan at the weekend said Ankara expected the first deliveries of the Russian system in the first half of July.