President Tayyip Erdogan stated on Sunday that the US had suggested selling F-16 fighter jets to Turkey in exchange for its participation in the F-35 program, which Ankara was kicked out of after purchasing Russian missile defense systems.

Turkey has asked the US to buy 40 Lockheed Martin F-16 fighter jets and roughly 80 upgrading kits for its existing warplanes, according to Reuters earlier this month.

Erdogan told reporters before leaving for a trip to West Africa that Turkey expects a return on its investment in the F-35 aircraft, and that talks are ongoing on the matter.

“There is the $1.4 billion payment we made for the F-35s, and the United States made such a proposition in exchange for these payments,” Erdogan explained.

“And in this regard, we said let’s take whatever actions are necessary to meet our country’s defense demands,” he added, adding that the new F-16 jets would contribute in the development of the fleet.

Ankara had bought over 100 Lockheed Martin F-35 fighter fighters, but the US pulled Turkey out of the deal in 2019 after it purchased 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.

Ankara’s purchase of the S-400s has also triggered U.S. sanctions. In December 2020, Washington blacklisted Turkey’s Defense Industry Directorate, its chief, Ismail Demir, and three other employees.

Since then the U.S. has repeatedly warned Turkey against buying further Russian weaponry. But Erdogan has indicated Ankara still intends to buy a second batch of S-400s from Russia, a move that could deepen the rift with Washington.

The request for the jets will likely have a difficult time getting approval from the U.S. Congress, where sentiment towards Turkey has soured deeply over recent years.

There is bipartisan support in U.S. Congress to push the Biden administration to put further pressure on Ankara, primarily over its purchase of Russian weapons and its human rights track record.

Ankara has said it hopes for better ties under U.S. President Joe Biden.