In an in-your-face rebuke to the United States, Turkey has announced that it is using American-made F-16 fighter jets to test the Russian S-400 anti-aircraft missiles that it bought from Moscow in defiance of U.S demands that it not activate the system.

The dispute resulted in the Pentagon banning Turkey from buying and helping to build the Lockheed Martin F-35 fighter jet.

While ostensibly the two NATO allies are trying to forge an agreement that would allow Turkey to rejoin the F-35 program, so far Turkey shows no sign that it is willing to give up the Russian system, which it has already bought, paid for, accepted delivery of, and is now testing.

The U.S. says operating the Russian system alongside the F-35 would compromise the fifth-generation fighter jet's stealthy flight profile.

Local Turkish media showed video of Boeing F-16 jets flying over the Turkish capital in what was announced as a test of the S-400 system in defiance of U.S. demands that the NATO ally not activate the anti-aircraft missiles.

“It's concerning,” Secretary of State Mike Pompeo said Tuesday at a briefing for reporters. “We have made very clear to the Turkish government our desire to see them move away from putting into full operationalization the S-400 weapon system.”

Meanwhile a spokesman for Turkish President Recep Tayyip Erdogan promised that Turkey would not integrate Russian S-400 missile systems into the NATO security or air defense systems and instead operate it as stand-alone platform, according to the state-run Anadolu Agency.

“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 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,” he said,

“I’ve been mandated to follow this with my U.S. counterpart Robert O'Brien."

“We are hopeful," said Pompeo. "We're still talking to the Turks. We're still trying to figure our way through this thing.”