Despite the protestations of Washington, Turkey, a NATO member, took delivery of its first shipment of the Russian-made S-400 air defense system in July. After a meeting with Erdogan on Wednesday, Trump told reporters that Ankara’s “acquisition of the S-400 creates some very serious challenges for us,” yet no agreement was reached on limiting or cancelling the purchase.

Erdogan said on Thursday that Trump had attempted to convince him to walk away from the deal with Moscow, but he refused. Trump’s proposal was “an infringement on our sovereign rights,” Turkish broadcaster NTV quoted Erdogan.

Erdogan’s decision to press ahead with the S-400 purchase saw it booted out of the F-35 fighter program in July. NATO officials were worried that jointly operating the alliance jet with the Russian air defense system could compromise the fighter’s stealth technology, while the Trump administration likely viewed the suspension as leverage to push Erdogan away from the Russian missiles.

Erdogan teased the possibility of buying Russian-made Sukhoi fighters in the F-35’s stead before departing for Washington. After meeting with Trump on Wednesday, Erdogan told reporters that Trump’s attitude to resolving the F-35 issue had become more “positive.”

Yet, as with the S-400, no specifics were announced