ANKARA — Turkey flatly rejected a proposal today by visiting US Congress members that it abandon its purchase of Russian S-400 air defense missiles in order to receive the F-35 fighter planes that it had ordered from the United States.

Speaking on a visit to Lithuania, Turkish Foreign Minister Mevlut Cavusoglu said the purchase of the Russian missiles would go through: “We have to protect our airspace. This is a must for us.”

The deal has annoyed Washington because the S-400 is not compatible with existing NATO missile systems. American legislators also support President Donald Trump’s demands for Turkey to drop the prosecution of Andrew Brunson, a US pastor who is under house arrest in Izmir while standing trial on charges of assisting Islamic and Kurdish militant groups.

Congress recently passed a bill that threatens to halt the sale of the F-35s to Turkey. Three members of the US Congress, led by Michael Turner, told Turkish legislators on Monday that Turkey should drop its missile purchase to win congressional support for the supply of the F-35s.