Turkey has to protect its airspace and the United States has failed to guarantee the sale of Patriot missile defense systems, Turkish Foreign Minister Mevlüt Çavuşoğlu said after a U.S. Congress delegation urged Ankara to give up purchasing S-400 missile systems from Russia to receive the F-35 fighter jets, Hurriyet reported.

“We have to protect our airspace. This is a must for us. They should understand it. Can the U.S. give us a guarantee to sell Patriots?” Çavuşoğlu said in a press conference in Lithuania on Aug. 28.

Çavuşoğlu also stressed international law regarding the U.S. reluctance to transfer F-35s to Turkey.

“We are a partner in the F-35 program and some parts are produced in Turkey,” he said, calling on the U.S. to “leave the language of threats” to solve the ongoing diplomatic crisis between the two countries.