Turkey and the U.S. have begun talks to resolve their differences in the military sector, the Turkish presidential spokesman said Friday.

In an interview aired on public broadcaster TRT, Ibrahim Kalin said the talks will be carried out at bilateral level and not under NATO supervision.

Turkey’s acquisition of the advanced Russian S-400 air-defense system prompted Washington to remove Turkey from the F-35 fifth-generation joint strike fighter program in July.

The U.S. maintains that the system could be used by Russia to covertly obtain classified details on the jet and is incompatible with NATO systems.

Read more: Turkey, US in talks to resolve F-35 jet, S-400 issues