Turkey has made a request to the U.S. to buy 40 Lockheed Martin-made F-16 fighter jets and nearly 80 modernization kits for its existing warplanes, as the NATO ally looks to modernize its Air Force after the purchase of F-35 jets fell through, Reuters reported citing sources familiar with the matter.

The deal, worth billions, is still working its way through the Foreign Military Sales process which is subject to approval by the U.S. State Department as well as the U.S. Congress which can block deals.

The people spoke on condition of anonymity to discuss the deal.

"As a matter of policy, the Department does not confirm or comment on proposed defense sales or transfers until they have been formally notified to Congress," a spokesperson for the State Department said. The Turkish Embassy in Washington declined to comment.

Ankara had ordered more than 100 F-35 jets, also made by Lockheed Martin Corp (LMT.N), but was removed from the program in 2019 after it acquired Russian S-400 missile defense systems.

The decades-old partnership between the NATO allies has gone through unprecedented tumult in the past five years over disagreements on Syria policy, Ankara’s closer ties with Moscow, its naval ambitions in the eastern Mediterranean, U.S. charges against a state-owned Turkish bank and erosion of rights and freedoms in Turkey.