Washington says it will cancel Turkey's purchase of Lockheed Martin F-35 stealth fighters if it proceeds with plans to buy the S-400 defense system.
"We won't co-locate those two assets, the S-400 and the F-35," NATO's Tod Wolters, a U.S. general, told Reuters at the Paris Airshow.
He said Shanahan and Akar were in close dialog about the issue, adding that the military-to-military relationship between the United States and NATO was "absolutely, positively solid"
Washington is beginning to wind down Turkey's participation in the F-35 program, and looking at imposing financial sanctions on Turkish firms beyond those that build parts for the fighter, top Pentagon arms buyer Ellen Lord said on Monday.
Turkish officials argue that Ankara is fulfilling its responsibilities in the F-35 project and that buying the S-400s is to meant its defense needs and poses no threats to the F-35.
U.S. officials say Russia could glean important information about the stealth characteristics of the F-35, the world's most advanced fighter.
Despite the dispute, U.S. and NATO officials are working to preserve their close cooperation with Turkey on military matters, mindful of Turkey's strategic location as a gateway to the Middle East.
Wolters said military exercises planned in Europe with the F-35 and other U.S. aircraft in coming months would continue as planned, but declined to say if any of those plans involved flying F-35s in Turkey.
U.S. Lieutenant General Steven Basham, deputy commander of U.S. Air Forces in Europe, told Reuters six U.S. F-15 fighter jets were participating in exercises in Turkey this week.
"We’re going to continue to work with our military counterparts in Turkey. Certainly these won’t be the last exercises that we have with Turkey," he said.
Wolters said there were also no plans to relocate a powerful early-warning AN/TYPY-2 radar stationed at Kurecik base in eastern Turkey as a result of the S-400 dispute.
"That’s not a concern at this time," Wolters said, noting that the radar would not be affected by the S-400 system.
U.S. officials continue to encourage Turkey to buy the U.S.-built Raytheon Patriot missile defense system - which is used by many other NATO allies - instead of the S-400.Industry sources said Turkey had been offered a generous technology package as part of its interest in the Patriot system.