Syrian pro-government forces have entered the key northern city of Manbij for the first time in six years, an army spokesman has said.
A Kurdish militia controlling the area withdrew and invited Syrian forces to retake the city amid fears that Turkish troops would launch a new offensive.
Turkey considers the Kurdish YPG forces to be part of a terrorist group.
The request by the US-backed Kurds follows 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.
Effort to deter Turkey
By Martin Patience, Middle East Correspondent, BBC News
The consequences of President Trump's surprise announcement to withdraw all US troops from Syria are now being seen on the ground.
Manbij is a strategically important town in northern Syria that until today was under the control of US-backed Kurdish forces.
The Kurds – who led the fight against IS – see Mr Trump's decision as a betrayal.
Without the support of the Americans, Kurdish leaders are now…
