A bipartisan group of lawmakers is urging the Pentagon to more quickly stop buying F-35 fighter jet components from Turkey.

Sens. James Lankford, R-Okla.; Jeanne Shaheen, D-N.H.; Thom Tillis, R-N.C.; and Chris Van Hollen, D-Md., complained in a letter to Defense Secretary Mark Esper on July 6 that the Pentagon’s plans to buy parts from Turkey into 2022 undercuts U.S. pressure on the country over its purchase of the Russian S-400 Triumf air defense system.

The U.S. formally removed Turkey from the multinational program in 2019 over the S-400 deal, and it ended training on the jet for Turkish pilots. Furthermore, the 2020 National Defense Authorization Act barred the transfer of F-35 aircraft to Turkey. The U.S. has warned that Turkey’s use of the S-400 could compromise the stealthy F-35.

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But Under Secretary of Defense for Acquisition and Sustainment Ellen Lord told reporters in January that it would allow prime contractor Lockheed Martin and engine-maker Pratt & Whitney to honor existing contractual obligations with Turkish manufacturers for F-35 components. That means Lockheed would receive Turkish parts through the end of Lot 14, with those planes set to be delivered to customers in 2022.

Read more: defense news