The Syrian army has entered the key northern city of Manbij for the first time in six years, an army spokesman has said.
However the US-led coalition operating in the area said it had not seen military changes inside Manbij.
A US official and a UK-based monitoring group said Syrian troops had deployed in the area around the city.
US-backed Kurds withdrew and invited Syrian forces to replace them amid fears that Turkish troops would attack.
Turkey considers the US-backed Kurdish YPG forces to be part of a terrorist group.
The Kurds’ request to the Syrian government followed the shock announcement that all US troops are to withdraw from Syria.
President Donald Trump announced the pullout of some 2,000 troops last week, asserting that the Islamic State group (IS) had been defeated.
But that claim has been disputed by important allies who say the move could lead to a resurgence of IS.
The Syrian army said it had raised a flag in Manbij. In a televised statement, a spokesman said the troops were there to “crush terrorism and defeat all invaders and occupiers”.
However a US official told Reuters that US troops had not changed their positions in Manbij and dozens still remained. The official said they had seen Syrian forces on the outskirts of the city but not inside it.
The US-led coalition against the Islamic State group said reports of changes to military forces in the city were “incorrect”.
Despite incorrect information about changes to military forces in the city of Manbij, Syria, #CJTFOIR has seen no indication that these claims are true. We call on everyone to respect the integrity of Manbij and the safety of its citizens. #DefeatISIS
— Inherent Resolve (@CJTFOIR) December 28, 2018
Nura al-Hamed, deputy head of the Manbij local authority, told AFP that the Syrian army deployment would “not enter the city of Manbij itself but will deploy on the demarcation line” with Turkish-backed Syrian rebels.
US and French coalition forces in the area remained at their positions and were continuing to conduct patrols, Ms Hamed told AFP.
Russia, which has been fighting on the side of Syrian President Bashar al-Assad since 2015, said the arrival of Syrian government troops would “help stabilise the situation”.
“The enlargement of the zone under the control of government forces… is without doubt a positive trend,” said Kremlin spokesman Dmitry Peskov.