The Pentagon announced on Friday that if by July 31 Turkey did not give up on the S-400 system, Ankara would be blocked from purchasing F-35 fighter jets.
President Recep Tayyip Erdogan on Wednesday said he hoped to persuade the United States not to exclude Turkey from taking part in the F-35 fighter jet programme, ahead of his meeting with President Donald Trump later this month.
Erdogan, who is to meet Trump on the margins of G20 summit in Japan on June 28-29, said in a televised speech that before the gathering he "would like to discuss this issue by telephone and reverse it from the current situation back to where we started".
Washington has given Turkey until the end of July to abandon its purchase of a major Russian missile defence system, which it considers incompatible with Ankara's participation in the US's construction of its F-35 stealth fighters.
The Pentagon announced on Friday that if by July 31 Turkey did not give up on the S-400 system, Ankara would be blocked from purchasing F-35 fighter jets and Turkish pilots currently training in the US would be expelled.
Erdogan vowed to seek answers on his country's "exclusion from F-35 project for reasons that have no rational or legitimate basis", adding that Turkey was not only a customer but also a programme partner of the F-35 project.
Turkey plans to buy 100 F-35s and its defence industry has made significant investment in the warplanes' production.
"We have so far paid $1.250 billion," Erdogan said.
The Turkish leader, however, repeated there would be no step back from Ankara's plans.
"Look, I am not saying Turkey will buy the S400 defence system, I say Turkey has bought it," he added.
"We have completed the business. God willing, the system will be delivered to our country next month."
The purchase has raised eyebrows among Turkey's NATO allies and provoked anger in Washington, which expected Ankara to opt for the American Patriot air defence system instead.
But Erdogan said Russia offered a better deal including reasonable pricing and a joint production promise.
"This is not an attack system but a defence system. Won't we take necessary measures to defend our country?" he said.
"Did we ask for such a defence system from America? Yes, we did. Did they deliver? No, they didn't," he said.
Acting US Defence Secretary Patrick Shanahan said Friday the US offer for the Patriots was "very competitive".
He also said he had sent a letter to his Turkish counterpart Hulusi Akar informing him of Washington's decision to pull Turkey out of the F-35 programme unless it abandons its plans to buy the S400 system.
Akar said in a statement Wednesday that the language used in the letter "does not bode well for the spirit of the alliance."