Washington says the S-400s pose a threat to its F-35 fighter jets and to NATO's broader defense systems. Turkey says it was unable to procure air defense systems from any NATO ally on satisfactory terms.
Erdogan said in an interview that aired on CBS News' "Face the Nation" on Sunday that nobody will be able to interfere in terms of what kind of defense systems Turkey acquires, from which country at what level.
The United States imposed sanctions on Turkey's Defense Industry Directorate, its chief, Ismail Demir, and three other employees in December following the country's acquisition of a first batch of S-400s.
Talks continued between Russia and Turkey about the delivery of a second batch, which Washington has repeatedly said would almost certainly trigger new sanctions.