Turkish TV showed amateur video purportedly showing the test firing of the Russian anti-aircraft weapon system by the Black Sea.
If verified, the test could stoke tensions between Turkey and the United States, which sharply opposed Ankara's purchase of weapons from Moscow on grounds they compromise shared NATO defense systems.
The US State Department wouldn't confirm the test but warned of "potentially serious consequences" if Ankara activates the weapon, with the threat of sanctions hanging in the background.
"If confirmed, we would condemn in the strongest terms the S-400 test missile launch as incompatible with Turkey's responsibilities as a NATO Ally and strategic partner of the United States," State Department spokesperson Morgan Ortagus said in an email.
"The United States has been clear on our expectation that the S-400 system should not be operationalized," she said.
As we reported earlier, the Turkish government shut off an area in the Black Sea, next to the city of Sinop, as the S-400 SAM tests approach.
Turkish Minute outlet claims that 10 Banshee air targets were delivered to the Sinop missile firing range for the tests.