Turkey requested in October to buy 40 F-16 fighters and nearly 80 upgrade kits for its existing F-16 fighter jets. Washington has so far refrained from commenting on the sale, claiming that it must go through the standard arms sales process.The sale of US weapons to NATO ally Turkey became contentious after Ankara acquired Russian-made defence missile systems, prompting US sanctions and excluding Turkey from the F-35 fighter jet program.First reported by Reuters, the March 17 dated letter was signed by the State Department's top legislative official, Naz Durakolu. She acknowledges the strained relations while calling Turkey's support for and defence ties with Ukraine "an important deterrent to malign influence in the region."While the letter does not provide any assurances or a timetable for sale, it emphasises that Washington's retaliatory actions in response to Ankara's purchase of Russian S-400 systems represent "a significant price paid."" The Administration believes that there are nonetheless compelling long-term NATO alliance unity and capability interests, as well as U.S. national security, economic and commercial interests that are supported by appropriate U.S. defence trade ties with Turkey," the letter stated."If the Department of State approves the proposed sale," it adds, "a Congressional notification will be required."The United States imposed sanctions on Turkey's defence industry following its purchase of the S-400s.Ankara had previously ordered more than 100 U.S. F-35 jets, but Washington removed Turkey from the program after buying the S-400s. Turkey has called the move unjust and demanded reimbursement for its $1.4 billion payment. Lockheed Martin Corp makes the F-35 and the F-16.