Turkey tested Russian-made S-400 air defense systems on US-made F-16 fighter jets in November, Russia’s news agency TASS reported, citing an unnamed source.

Fighter Jets World, a military aviation website that tracks news about fighter jets, reportedly published a video from 2019 by Turkey’s TRT channel showing the missile systems being tested on the US-made fighter aircraft, according to TASS.

“Yes, such trials have indeed taken place last November,” an unnamed source told the Russian news agency.

Turkey’s purchase of Russian S-400s has been a point of conflict with the US, which had in the past urged Ankara to drop the system, adding that it was not compatible with NATO defenses.

However, Turkey has said that it had acquired the missiles in to test communication between the platforms to ensure it did not accidentally shoot down its own jets.

The missile defense systems were expected to be operational by April, but there has been no sign of such a move so far.