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.

Worse still, because the S-400 system wasn’t designed to play well with Western assets, it could pose a direct threat to allied aircraft operating near Turkish airspace. Without being connected to NATO’s shared network, the S-400 system would not be able to differentiate between friendly and enemy aircraft easily, thereby increasing the likelihood that the Turkish military could accidentally shoot down allied planes.

“We believe that the S-400s will not pose any threat to the F-35 program. We do not want the F-35 program to be jeopardized by anything,” said the Turkish presidential spokesman Ibrahim Kalin at a forum in Berlin. “We will be talking about this next week at the NATO summit, we believe that those concerns can be addressed.”

While the F-16 has already been operational for decades and there may not be any truly damning bit of intel to be gleaned from these exercises, Ankara’s behavior can also be read as a message that it has made up its mind regarding its preference of the S-400 over the F-35. These tests will likely result in new sanctions being placed on the NATO ally, and will likely further escalate tensions between the U.S. and Turkey.

After all, one of NATO’s main objectives is curbing the aggressive behavior or Russia — a nation Turkey finds itself increasingly friendly toward.