Turkey will not back down from its decision to buy Russian S-400 missile-defense systems despite a U.S. threat that Ankara will be excluded from the F-35 fighter jet program, Foreign Minister Mevlut Cavusoglu said on June 13.

Cavusoglu’s statement came a day after President Recep Tayyip Erdogan said he hoped to persuade the United States to back off its threat.

Erdogan is scheduled to meet with President Donald Trump on the margins of a Group of 20 summit in Japan on June 28-29, but the Turkish leader said he will speak to the U.S. president by phone on June 13.

Erdogan on June 12 said he “would like to discuss this issue by telephone and reverse it from the current situation back to where we started.”

The United States has given Turkey a deadline of July 31 to reverse its purchase of the Russian-made S-400 missile-defense system or face the loss of subcontracting work on the U.S.-led F-35 fighter jet project. Ankara also plans to buy 100 of the jets for its own military’s use.

Acting U.S. Defense Secretary Patrick Shanahan sent a letter to his Turkish counterpart that explained the measures to remove Turkey from the program unless Ankara changes course.

In what was Turkey’s first direct response to the letter, Cavusoglu said no one can give Turkey ultimatums.

“Turkey will not back down from its decisions with these kinds of letters,” he said. “Turkey bought the S-400. It is going to be delivered and stationed in Turkey.”

Washington and its allies consider the missile system to be incompatible with NATO systems and a threat to F-35 fighter jets.

Russia on June 11 said it plans to deliver its S-400 missile-defense systems to Turkey in July.

Erdogan has resisted U.S. pressure to back out of the Russian purchase, saying it is a done deal and has dismissed suggestions it is a threat to NATO systems.