Turkey has warned the U.S., its NATO ally, of swift retaliation against an “unacceptable threat” of sanctions after Ankara began taking delivery of new Russian S-400 missile defenses.

Turkey began receiving the surface-to-air S-400 systems earlier this month, prompting the United States to begin removing it from its F-35 stealth fighter program over security concerns.

The S-400 is the latest generation surface-to-air defense system developed by Russia as a rival for America’s own Patriot weaponry, and is considered by NATO countries to pose a threat to their combined air operations.

“If the United States portrays an adversarial attitude towards us, we will take retaliatory measures, as we’ve told them. This is not a threat or a bluff,” Foreign Minister Mevlut Cavusoglu said in an interview with broadcaster TGRT Haber, Reuters reports.

“We are not a country that will bow down to those who show a animosity towards Turkey,” he said, reiterating a threat of retaliation that Turkey made last month.

Cavusoglu cautioned he did not expect the U.S. administration to take such action.

“Trump does not want to impose sanctions on Turkey and he frequently says that his administration and the previous U.S. administration is also responsible for Turkey not being able to buy Patriot systems. This is true,” Cavusoglu said.

The S-400 purchase is one dispute fuelling tensions between two nations also at odds over U.S. support for Syrian Kurdish militias which Ankara brands as terrorists and Turkish backing for Venezuela.

Turkey had planned to buy 100 F-35A fighter jets, with pilots already training in the United States.

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