Turkey's plans to buy the Russian S-400 missile defence system would give a weapon used by "known foes" of NATO deep insight into the radar-evading F-35 fighter jets arriving in growing numbers in Europe, the top U.S. Air Force general in Europe said.
General Tod Wolters, also the NATO Allied Air Commander, told Reuters news agency the issue was worrying, but he was working to maintain strong military ties with the NATO member for now.
"Anything that an S-400 can do that affords it the ability to better understand a capability like the F-35 is certainly not to the advantage of the coalition," said Wolters, who was in Britain for an air chiefs conference in London and the Royal International Air Tattoo at RAF Fairford.
Turkey's plans to buy the Russian air and missile defence system have raised tensions with Washington, and U.S. lawmakers are seeking to block the transfer of any F-35 jets to Turkey.
A senior U.S. official last month said Turkey was a key NATO ally, but its purchases of the F-35 would be at risk and Ankara would face sanctions if it proceeded to buy the S-400.