The United States will spare Turkey from sanctions over its purchase of the S-400 missile system from Russia if it does not turn it on, a US official said Friday. 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 heavens sake, dont turn this on," a State Department official told reporters on condition of anonymity. "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," he added. "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. Batteries of the S-400 began arriving in Turkey in July but have not yet been switched on, which the US official acknowledged. Turkey and the United States have been at loggerheads over the purchase of the S-400 system, which the United States says is not compatible with NATO defenses and poses a threat to Lockheed Martin’s F-35 ‘stealth’ fighter jet. Washington has previously warned Ankara that it will face sanctions over the purchase and removed Turkey from its F-35 program, but has fallen short of slapping such penalties.