(Photo: Vitaly Nevar/Reuters)

Shanahan told journalists that Washington remained its opposition to Turkey's adoption of the S-400 anti-aircraft technology of Russia. He said the US will move work out of Turkey if it decides to buy the S-400. Shanahan added that he had met with delegations from US aerospace manufacturers Lockheed Martin and United Technologies to discuss options if Turkey refuses to forego the S-400.

Washington has warned for months that Turkey's adoption of Russian missile technology alongside US fighter jets would pose a threat to the F-35 and endanger Western defense. As a member of NATO, Turkey is taking part in the production of the fighter jet for use by members of the treaty, and has plans to buy S-400, saying the S-400 would not pose threat to the jets and Ankara had taken measures to prevent that.