'Real risk' of Turkey sanctions over Russian arms: US
"That risk is very real because they... continue to pursue the S-400," said R. Clarke Cooper, the top State Department official in charge of arms sales.
"And, of course, with the testing of it, sanctions is very much something that is on the table," he told a small group of reporters.
Turkey has confirmed the first test of the S-400 despite months of warnings from the United States that the system was incompatible with membership in NATO and could help Russia hone skills in shooting down Western fighter jets.
A law passed overwhelmingly by Congress in 2017 lays out sanctions against nations that buy significant quantities of arms from US adversaries including Russia.
The US had already kicked out Turkey from its elite F-35 fighter-jet program after it bought the S-400 system in 2017.
Cooper said the US had set a red line of Turkey not activating the S-400.
Even after the test, he hoped that Turkey would "walk back from operationalising" the system.
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"We certainly are working to make sure that Turkey remains in the West. This is something that's important not only to the United States, but to the overall alliance."
Turkish President Recep Tayyip Erdogan in a speech Sunday hit back at the threat of sanctions, accusing the US of treating his country like a "tribal state."
The S-400 purchase had been seen as part of Erdogan's effort to rebalance Turkey's relationship closer to Russia amid tensions with Western powers on multiple fronts.
But relations between Turkey and Russia are hardly smooth, with Erdogan condemning Moscow over an air strike Monday that killed dozens of pro-Ankara rebels in Syria.
US President Donald Trump had earlier appeared to show sympathy for Erdogan, who blamed his decision to turn to Russia on Trump's predecessor Barack Obama.