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

Axar.az reports citing Turkish media.

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.

Turkey, however, says that the S-400 would neither be integrated into NATO systems, nor pose a threat to the alliance.

“The two presidents appointed me and [U.S. National Security Advisor] Robert O’Brien at the White House meeting. We are starting to work on it as I speak,” said Kalin.

“S-400 system can be used independently without being integrated into NATO defense system,” he added.

Kalin also said Russian President Vladimir Putin was expected to visit Ankara in the first half of January.

“There might be a ceremony for the completion of the Turk Stream project. Also, bilateral relations and regional issues will be discussed,” he added.