The decision has injected new uncertainties into the eight-year long Syrian war and a flurry of contacts over how a resulting security vacuum will be filled across a swathe of northern and eastern Syria where the US forces are stationed.
Zakharova also said Russian Federation remains committed to an agreement it had struck with Turkey to stabilize a de-escalation zone in Syria's Idlib province, but Moscow is anxious by an increase in the number of ceasefire violations there.
Adding to the confusion, a military spokesperson said the U.S. had already begun "the process of our deliberate withdrawal" from Syria.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements", Ryan said in a statement to the Associated Press.
Witnesses on the ground told the British-based Syrian Observatory for Human Rights they saw 10 armored vehicles, and other equipment, roll out from a US base in Rmeilan in Hassakeh province and head toward neighboring Iraq.
The US-led coalition has several other bases across northeastern Syria, as well as in neighboring Iraq, where Trump has said American forces will remain.
Trump's decision to withdraw from Syria spurred confusion and criticism over what many people perceived as the abandonment of local Kurdish allies who were facing threats of attack from Turkey.
Recent reports have indicated the U.S. is considering leaving some troops in place at the al-Tanf base in southern Syria, which is seen as a key bulwark against Iran. Many are also anxious that the Kurdish forces, which spearheaded the fight against the "Islamic State" militants with USA backing, would be left vulnerable to an imminent attack by Turkey.
Meanwhile, US National Security Adviser John Bolton on Sunday outlined conditions for US troop departure from Syria, including a provision that Turkey should guarantee the safety of YPG Kurdish militants in Syria.
Pompeo has also sought to reassure the Kurds that they will be safe after USA troops withdraw from the country, The Associated Press reported.
"These have been folks that have fought with us and it's important that we do everything we can to ensure that those folks that fought with us are protected", Pompeo said of the Kurds while visiting Erbil, the capital of Iraq's semi-autonomous Kurdistan region, after talks in Baghdad.
United States defense officials said the withdrawal was only of certain types of gear, and not troops.
Residents near Iraqi border crossings that are typically used by USA forces going in and out of Syria said they had seen no obvious or large-scale movement of American ground forces on Friday.
US forces' vehicles and structures are seen on the outskirts of the northern Syrian town of Manbij on December 26, 2018. Once the troops are gone, the US will have ended three years of organizing, arming, advising and providing air cover for Syrian, Kurdish and Arab fighters in an open-ended campaign devised by the Obama administration to deal the IS group a lasting defeat. The first of those criteria is unmet, but feasible, as the terror group now holds only a small patch of land in Syria.
"Obviously when there is a political solution, we will withdraw", he said without elaborating.
On December 19, Trump said he had ordered the withdrawal of all United States forces in Syria, which are believed to number around 2,000.
The U.S. decision has injected new uncertainties into the eight-year long Syrian war and a flurry of contacts over how a resulting security vacuum will be filled across a swathe of northern and eastern Syria where the U.S. forces are stationed.