In what has become the main source of tension between Ankara and Washington, the NATO allies have sparred publicly for months over Turkey’s purchase of the S-400s, which Washington has said could trigger US sanctions.

US Acting Secretary of Defense Patrick Shanahan last week sent his Turkish counterpart a letter warning that Ankara would be pulled out of the F-35 jet program unless it changes course from its plans to install the defenses.

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 S-400, it is going to be delivered and stationed in Turkey.”

The S-400s are not compatible with NATO’s defense systems and Washington says they would compromise its F-35s, which Turkey also plans to buy. Turkey has proposed that the allies form a working group to assess the impact of the S-400s, but has yet to receive a response from the United States.

Cavusoglu on Thursday repeated Turkey’s call for the joint working group, saying experts from both countries should come together to evaluate US concerns, Reuters reported.

A day earlier, Turkish President Tayyip Erdogan said that Turkey had completed the deal with Russia and that the systems will be delivered in July. Russia has said it will begin the delivery of the systems in July.

Erdogan also said that Ankara would challenge its potential removal from the F-35 program on every platform and hold those who exclude Turkey accountable.

The United States has threatened to impose sanctions on Ankara under its Countering America’s Adversaries Through Sanctions Act (CAATSA), the possibility of which has spooked investors and caused a selloff in the lira this year.

While Turkey has dismissed the US warnings, Washington has said discussions are taking place with Ankara on selling Turkey rival Raytheon Co Patriot defense systems. But, Erdogan has said the US offer was not “as good as the S-400s”.

Turkish Defense Minister Hulusi Akar has said Turkey is working on a response to Shanahan’s letter and that it will be delivered in coming days.

The ministers later spoke by telephone on Thursday and discussed the letter, Turkey’s defense ministry said, adding that Akar had “emphasized the improper wording not in line with the spirit of the Alliance” in the letter during the call.

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