While Turkey’s decision to purchase Russia’s advanced S-400 air defense system may have cost them access to America’s F-35 Joint Strike Fighter, it’s are already moving ahead with testing the Russian weapon system against U.S. made F-16s.

Some American officials have already voiced concerns over the possibility that Russia will gain access to target acquisition data derived from Turkish operated S-400s, effectively giving the Russian military access to a treasure trove of data relating to one of America’s top fourth generation multi-role fighters.

“Erdoğan is thumbing his nose at Trump, the U.S. [and] NATO, and crossing another red line on S-400s,” Democratic Sen. Chris Van Hollen of Maryland said on Twitter.

Turkey has announced that it intends to operate its S-400 systems as entirely independent air defense platforms, outside the scope of NATO security or air defense systems. Nonetheless, concerns persist that Turkey’s use of Russia’s most advanced systems could compromise systems employed by NATO, as Turkey now has access to both and has clearly demonstrated an unwillingness to yield to U.S. pressure regarding the applications of each.