President Recep Tayyip Erdogan on Friday said Turkey will look to Russia for sophisticated new air defense systems “in the shortest time,” a purchase that has alarmed Ankara’s NATO partners.

“Turkey needs S-400s and its deal has been done,” AFP reports Erdogan said in the western city of Balikesir. “God willing, we will buy them in the shortest time.”

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 aircraft.

Turkey flagged the possibility of the buy last September, with President Erdogan appearing to mock Western nations concerned with the deal, joking that “they went crazy” over Russia taking yet another step towards bringing Turkey into its orbit.

Washington has long warned Ankara against acquiring S-400s from Russia, saying the prospects for future Turkish military-industrial cooperation with the U.S. would be damaged.

“One of the things that we want is it’s important for NATO countries to have military equipment that’s considered interoperable with the NATO systems, with the systems that NATO nations currently have,” spokeswoman Heather Nauert told reporters last September.

“A Russian system, if Turkey were to buy these S-400s, as is being reported, that would not meet that standard, so that would, of course, be a concern of ours.”

The S-400 missile deal comes as Turkey continues its fight with the U.S., its NATO-ally, over Ankara’s detention of American pastor Andrew Brunson on terror-related charges.