Turkey and the United States havesparred publicly for months over Ankara's order for the S-400s, which are notcompatible with NATO's systems.
"Turkey has already boughtS-400 defence systems. It is a done deal. I hope these systems will bedelivered to our country next month," Erdogan said.
U.S. acting Secretary of DefensePatrick Shanahan outlined last week how Turkey would be pulled out of the F-35fighter jet programme unless Ankara changed course from its plans to purchasethe S-400 missile defence system.
Turkey has criticised the letterfrom Washington and said it did not live up to the spirit of the NATO alliance.It is working on a response to be sent in the coming days, according to TurkishDefence Minister Hulusi Akar.
The United States says Turkey'sacquisition of the Russian system poses a threat to Lockheed Martin Corp's F-35stealth fighters, and has warned of potential U.S. sanctions if Ankara presseson with the deal.