President Donald Trump on Wednesday lifted sanctions on Turkey that had been imposed over its assault on US-allied Syrian Kurdish fighters, which created a crisis between the NATO allies.

But tensions remain, with Turkey still facing punishment for buying the S-400 missile defense system from Russia despite its membership in the Western alliance.

"There are probably cooler heads in Ankara that are saying, for heaven's sake, don't turn this on," a State Department official told reporters on condition of anonymity.

The official said it did not appear that Turkey had activated the S-400, which NATO powers fear could help Russia hone its skills in shooting down Western planes and missiles.

"There is still work to get the Turks to walk away from the S-400 -- be it turn it off, send it back, destroy it, what have you," the State Department official said.

"Ideally they should never have acquired or received any component of the S-400, but now that that line has been crossed, it is a matter of how to isolate, compartmentalize that -- neutralize it -- and move forward," he said.

Turkish President Recep Tayyip Erdogan has nonetheless kept building relations with Russia, which agreed to deploy forces in northern Syria to replace US troops pulled out by Trump.

Senator Lindsey Graham, a close ally of Trump, has said that he had made the pitch directly to Turkey to avoid sanctions by not activating the S-400.

Under a 2017 law, the United States is required to impose sanctions on any country if they make "significant" military transactions with US adversaries including Russia.