The withdrawal began with shipments of military equipment, USA defense officials said.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements", he said.
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". It said a convoy of about 10 armored vehicles, in addition to some trucks, pulled out from Syria's northeastern town of Rmeilan into Iraq.
Later in the day, however, some of those outlets conceded that the spokesman did not indicate whether any American service members were being pulled out or whether the USA military was carrying out other activities that could be considered a withdrawal.
Turkey's defense minister on Friday pledged to wage a campaign against a USA -backed Syrian Kurdish militia, sharpening focus on a potential conflict the United States has sought to prevent.
Russia, which has deployed forces into Syria in support of the Damascus regime, said it had the impression that the United States wanted to stay despite the announced withdrawal of U.S. troops, RIA news agency reported.
Secretary of Defense James Mattis had resigned immediately after Trump's claim in December that the Islamic State group had been defeated. His statements appeared to contradict those made earlier by Trump.
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The start of the drawdown coincided with a Middle East tour by US Secretary of State Mike Pompeo, who insisted in Cairo on Thursday that the withdrawal would go ahead despite widespread criticism.
Trump also refused to directly answer a question about the timeline of the USA withdrawal from Syria, saying only that it will happen "over a period of time".
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 USA withdrawal plans.
Turkey views the YPG as an extension of the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade insurgency in Turkey's largely Kurdish southeast.
On the one hand, Turkey aims to pursue a campaign against Kurdish forces that have allied with the United States, and on the other the Russia- and Iran-backed Syrian regime sees the chance to recover a huge chunk of territory. He spoke before the announcement on the US withdrawal and did not address it.
File image of USA troops in Syria.
"I can not share your confidence that they are leaving there because we never saw an official strategy", she added.