A Trump Administration official says the military has started withdrawing ground equipment from the region in recent days.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements", he added.
This follows a week of speculation about the process, and comes just a day after Pentagon officials said that there had been no changes to the timeline, despite John Bolton's weekend talk of making the pullout "conditional".
Ankara has been infuriated by the US support for the Syrian Democratic Forces (SDF), which is dominated by the YPG under the pretext of fighting against Daesh.
Turkey says the weapons are ultimately transferred to the PKK - designated as a terror group by the U.S., Turkey and the European Union - and used against Turkey. The plan hinges on Turkish cooperation to secure a swathe of northeast Syria as the United States departs. Trump's abrupt decision in December to pull them out, declaring in a tweet the defeat of IS, sent shockwaves across the region and a flurry of criticism from some of his generals and national security advisers, and led to the resignation of U.S. Defense Minister James Mattis and the top U.S. envoy to the anti-IS coalition. His statements appeared to contradict those made earlier by Trump.
U.S. Secretary of State Mike Pompeo also addressed the issue in a speech in Cairo on Thursday.
After initially tweeting about the decision to bring back USA troops "now", Trump this week said "we will be leaving at a proper pace while at the same time continuing to fight ISIS and doing all else that is prudent and necessary!"
Maria Zakharova, a Russian Foreign Ministry spokeswoman, said it was important for Syrian Kurds and the Syrian government to start talking to each other in light of the U.S. withdrawal plans.
They have also argued that it would further allow Syrian regime ally Iran to extend its influence across the country and potentially threaten Israel.
Since his surprise announcement last month, Trump has stressed any withdrawal would be coordinated, gradual and "prudent".
Russia's Foreign Ministry spokeswoman Maria Zakharova said the Americans are not serious about withdrawing from Syria.
"I can not share your confidence that they are leaving there because we never saw an official strategy", she added. The U.S. provided military training and supplied truckloads of weapons to the YPG, disregarding warnings from Ankara that the YPG is organically linked to the PKK, and partnering with one terrorist group to fight another was not acceptable.