Syrian government troops have moved into a series of towns and villages in northern Syria, setting up a potential clash with Turkish-led forces in the area, as US troops prepared to pull out.
The Syrian army's deployment on Monday near the Turkish border came hours after Syrian Kurdish forces previously allied with the US said they had reached a deal with President Bashar Assad's government to help fend off Turkey's invasion, now in its sixth day.
The announcement of a deal between Syria's Kurds and its government is a major shift in alliances that came after President Donald Trump ordered all US troops withdrawn from the northern border area amid the rapidly spreading chaos.
The shift sets up a potential clash between Turkey and Syria and raises the spectre of a resurgent Islamic State group as the US relinquishes any remaining influence in northern Syria to Assad and his chief backer, Russia.
The fighting also seems likely to endanger, if not altogether crush, the brief experiment in self-rule set up by Syria's Kurds since the war began.
Syrian troops arrived on Monday in the northern province of Raqqa aboard buses and pickup trucks with mounted heavy machineguns.
Turkey has pressed on with its invasion of northern Syria, warning its NATO allies in Europe and the United States not to stand in its way.
Turkish troops and Syrian proxy forces have steadily pushed their way south of the border, clashing with the Kurdish fighters over a stretch of 200 kilometres. The offensive has displaced at least 130,000 people.
Turkey's president signaled that it was ready to launch an assault on the city of Manbij, where Kurdish-led groups invited Syrian government forces to re-enter and defend the town.
"We are about to implement our decision on Manbij," President Recep Tayyip Erdogan told reporters on Monday.
He added that Turkey's aim would be to return the city to Arab populations whom he said where its rightful owners.
Turkish forces were already positioned at the city's edge, according to CNN-Turk.
Erdogan has already said Turkey will not negotiate with the Syrian Kurdish fighters, which it considers "terrorists" for links to a long-running Kurdish insurgency within its own borders.
Syrian state media reported late Sunday that government troops were marching toward Manbij - as well as the border town of Kobani that in 2015 witnessed the Islamic State group's first defeat in a battle by US-backed Kurdish fighters.
In another direction, Syrian troops moved east from Aleppo province to Raqqa where state media said they had reached Ein Issa. Heavy fighting the previous day there reached a Kurdish-run displaced-person camp that is home to some 12,000 people, including around 1000 wives and widows of IS fighters and their children. Hundreds are believed to escaped amid the chaos.
The Syrian army also moved into the town of Tal Tamr, which is about 20 kilometres from the Turkish border, and Tabqa, known for its dam on the Euphrates River and its nearby air base that carries the same name.