Erdogan and Trump held talks at the White House to overcome mounting differences between the allies. Washington has said the S-400s pose a threat to its F-35 stealth fighter jets and has suspended Turkey from the jet program, where it is a buyer and manufacturer. It has also warned of U.S. sanctions over the deal, but has yet to impose them.
Speaking to members of his AK Party in parliament, Erdogan also said he had told Trump that if the United States maintained its "uncompromising stance" on the F-35s, Turkey would have to seek alternatives to meet its medium-term defense needs.