Syrian pro-government forces have entered the key northern city of Manbij for the first time in six years, an army spokesman has said.
The Kurdish militia that controlled the area asked for help from the government 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 being forced to choose what they regard as the least bad option.
They will be hoping that the presence of the Syrian government army – which is backed by Russia and Iran – will deter Turkey from launching an attack.
What’s the context?
Turkey has been intensifying its military activity near positions held by Kurdish fighters since President Trump’s withdrawal announcement on 19 December.
US soldiers have been working closely with Kurdish forces who form part of an alliance – the Syrian Democratic Forces (SDF) – in the battle against IS.
But this policy has infuriated Turkey, which is battling the militia in the nearby city of Afrin.
It also says it is frustrated over what it sees as delays to a deal agreed with the US to clear Manbij of Kurdish fighters.
The deal over the city, which lies to the west of the Euphrates river, was agreed in February in a bid stabilise the region.
The US also has soldiers in Manbij, which was taken from IS by forces led by the Kurdish YPG in 2016.
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Tensions in the region have risen in recent weeks, and Turkish President Recep Tayyip Erdogan has threatened to launch a new offensive against the Kurds.
Turkey has launched two previous offensives against the Kurds in Syria since 2016.
Mr Erdogan wants to prevent the Kurds from consolidating their hold on Syrian territory and forming an autonomous region on Turkey’s border.
Turkey considers the militia to be an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in south-eastern Turkey for three decades.