Turkish President Recep Tayyip Erdogan says his country’s purchase of Russia�s S-400 is a “done” deal, and that the delivery of the missile defense system may happen sooner than the date set.

Erdogan made the remarks to reporters on his plane while returning from a trip to Russia, Turkey’s Daily Sabah newspaper reported on Wednesday.

The Turkish president said that he and his foreign minister were frequently asked about Ankara’s decision on the purchase of the S-400 system from Moscow.

“We answer that ‘this deal is done, everything is settled.’ The delivery of the S-400 missile defense system was to be in July. Maybe it can be brought forward,” he emphasized.

Erdogan also noted that Turkey’s requests for defense support had not received positive responses, leading Ankara to find its own alternatives.

He further accused the US of having double standards on Russian S-400 while countries such as Greece, Bulgaria and Slovakia have similar defense systems.

“Why shouldn’t one of NATO’s strongest militaries have a strong air defense system?” Erdogan asked.

Turkey has signed a $2.5 billion deal with Russia for the purchase of S-400 batteries with the delivery set to begin in July. It has, at the same time, ordered 30 of the US F-35 stealth fighter jets and plans to buy over 100 in total.

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But Ankara’s military deal with Moscow has angered Washington, with the latter issuing an ultimatum for Turkey to stop buying the Russian system or risk losing its NATO membership.

Recently, the Pentagon halted the delivery of equipment related to the F-35 to Turkey. Leaders of the US Senate Foreign Relations and Armed Services Committees warned Turkey on Tuesday that it risked tough sanctions if it pursued plans to acquire S-400.

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“By the end of the year, Turkey will have either F-35 advanced fighter aircraft on its soil or a Russian S-400 surface-to-air missile defense system. It will not have both,” Republican Senators Jim Risch and Jim Inhofe and Democratic Senators Bob Menendez and Jack Reed said in a New York Times opinion column.

Russia’s S-400 system is designed to engage aerodynamic targets at a range of up to 400 kilometers and ballistic missiles up to 60 kilometers away. It can also deal with up to 36 aerial targets simultaneously.