(Bloomberg) -- Turkish President Recep Tayyip Erdogan wants to use an upcoming global summit to lobby U.S. President Joe Biden to allow Ankara to buy dozens of American warplanes, in a bid to overcome Washington’s resistance to major arms deals with his country following its purchase of Russian air defenses.

Turkey sent a formal request to the U.S. on Sept. 30 to purchase 40 new F-16 Block 70 aircraft and nearly 80 kits from Lockheed Martin Corp. to modernize its existing F-16 fighters, two Turkish officials familiar with the matter said. The deal is potentially worth $6 billion, they said, but approval will be difficult to win, given Congress’s opposition to the Russian S-400 missile purchase and Turkey’s own uncompromising stance. 

The Turkish officials said Erdogan expects to meet Biden during the Group of 20 nations summit in Rome at the end of this month, though no meeting has been announced and it’s unclear how willing Biden might be to entertain the weapons request. Turkey’s aim is to secure jets compatible with NATO, they said, speaking on condition of anonymity to discuss strategic matters.

A State Department spokesperson, who asked not to be identified, said the department doesn’t comment on proposed defense sales. The White House didn’t immediately respond to a request for comment, and the National Security Council referred questions to the State Department. 

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The Turkish president so far hasn’t signaled any progress toward resolving the clash with the U.S., which worries the S-400 could be used to collect intelligence on the stealth capabilities of the American F-35 fighter jet, which Turkish companies had helped to build and Ankara had planned to buy before the rift over the Russian missiles. 

Washington has demanded that Ankara scrap the S-400 in return for the lifting of related U.S. sanctions, but Turkey has shown no inclination. The sanctions cut off Turkey’s top defense procurement agency from U.S. financial institutions, military hardware and technology. New export licenses to transfer American goods or technology to the agency have been banned.

“Congress has said that we’re not going to sign off on major arms agreements with Turkey until we get resolution on the S-400s,” Aaron Stein, the director of the Middle East program at the Foreign Policy Research Institute in Washington, said in a recent podcast. “This letter or request that the Turks have put forward faces a very tricky task to get through to get approval for the export of F-16s to Turkey.” 

Ankara had wrangled with Washington for years over access to its Patriot missile system, with Washington balking at Turkish demands for a transfer of technology.

Turkey, already overdue in retiring its F-4 jets, is planning to upgrade its F-16 fleet as a stopgap solution to its fighter capacity as it aspires to develop its own jets, according to Arda Mevlutoglu, a Turkish aviation expert based in Ankara. 

“If Turkey can win U.S. approval for the sale of the new F-16 Block 70 aircraft and upgrade kits, that would come as a huge relief to the air force,” Mevlutoglu said on Monday. “Those planes would probably fulfill the capability until the 2030s, giving Turkey two critical decades to try and develop its own fighters.”

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