"Timetables or the timing of the withdrawal occurs as a result of the fulfillment of the conditions and the establishment of the circumstances that we want to see", Bolton said Sunday, which struck a different tone than the December 19 announcement White House Press Secretary Sarah Sanders made, in which she said the USA had already begun pulling American troops.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements".
The withdrawal appeared to have begun Thursday night, according to the Syrian Observatory for Human Rights, which monitors the conflict in Syria through activists on the ground.More news: Woman in vegetative state who gave birth was 'raped several times'
It said a convoy of ten armoured vehicles and some trucks had pulled out of Rmeilan, in Syria's north west, into Iraq.
U.S. Secretary of State Mike Pompeo, who has been touring the Middle East this week to reassure allies of Washington's commitment to regional security, said on Thursday the withdrawal would not be scuppered despite the Turkish threats.
The announcement that U.S. troops would leave the civil war-racked country - where they have been deployed to assist in the multinational fight against the Islamic State (IS) jihadist group - shocked global partners and American politicians alike.
Russia's foreign ministry said Friday that it suspected the US government was really intent on maintaining a military force inside Syria indefinitely, in spite of the pullout officially being announced by the Americans. Top U.S. officials were shocked too, among them Defense Secretary Jim Mattis, who quit in protest. Turkey views the dominant Syrian Kurdish groups as a national security threat.More news: South African teacher suspended over seating students by race
The White House has not yet publicly altered it's most recent criteria, those announced by Bolton a week ago: that ISIS must be defeated and the Kurdish forces' security guaranteed before the American troops all come out.
Russia's Foreign Ministry spokeswoman Maria Zakharova said the Americans are not serious about withdrawing from Syria.
Addressing concerns that the United States withdrawal would allow Iranian and Iran-backed forces to remain in Syria and fulfill Tehran's ambition of establishing a strategically important land bridge to the Mediterranean, Mr Pompeo said the United States "will use diplomacy and work with our partners to expel every last Iranian boot" from the country.
Foreign Minister Mevlut Cavusoglu went so far on Wednesday as to say that if the USA withdrawal from Syria was delayed, "then we will put this decision into practice", suggesting Turkey could go after the Kurds in short order.More news: Thunder C Nerlens Noel Released From Hospital After Fall, Diagnosed With Concussion