Turkey says they will test Russian S-400 defense system despite of US pressure.

As part of the Air Defense System project, the Ankara Governor’s Office on Nov. 24 announced that high- and low-altitude test flights would be performed by F-16s and other aircraft on Nov. 25 and Nov. 26.

Ankara’s purchase of the S-400s has been a major factor souring relations with the United States, which says the system is not compatible with NATO defenses and poses a threat to Lockheed Martin’s F-35 stealth fighter jets.

The provincial governor’s office announced on Sunday that the Turkish Air Force F-16s and other aircraft will conduct low- and high-altitude flights over Ankara on Monday and Tuesday to test an air defense system project.

Trump said their talks were “wonderful” but it was unclear if the two NATO allies made any breakthrough on the S-400 issue and Erdogan subsequently said U.S. pressure to get rid of the S-400s was an infringement of sovereign rights.

Washington has suspended Turkey from the F-35 program, which it was a buyer and producer of, to punish it for the S-400 purchase. It has warned of possible U.S. sanctions over the deal but has not yet imposed them.

Trump has told Erdogan the United States is ready to sell Ankara U.S. Patriot systems if it drops the Russian system.

The U.S. maintains that the system could be used by Russia to covertly obtain classified details on the jet and is incompatible with NATO systems.

Turkey, however, says that the S-400 would neither be integrated into NATO systems, nor pose a threat to the alliance.