Neighbouring Turkey, as well as some officials in Syria, said late on Friday the deadline could be extended.
Government troops have seized swathes of northern and eastern territory in the last two weeks from the Kurdish-led Syrian Democratic Forces in a rapid turn of events that has consolidated President Ahmed al-Sharaa's rule.
Sharaa's forces were approaching a last cluster of Kurdish-held cities in the northeast earlier this week when he abruptly announced a ceasefire, giving the SDF until Saturday night to come up with a plan to integrate with Syria's army.
As the deadline approached, SDF forces also reinforced their defensive positions in the cities of Qamishli, Hasakeh and Kobane for a possible fight, Kurdish security sources told Reuters.
Syrian officials and SDF sources said it was likely the Saturday deadline would be extended for several days, possibly up to a week.
"Extending the ceasefire for a little longer may come onto the agenda," said Hakan Fidan, the foreign minister of Turkey, which is the strongest foreign backer of Sharaa's government and sees the SDF as an arm of the outlawed Kurdistan Workers Party, or PKK.
The possible showdown in northern Syria is the culmination of rising tensions in the past year.
Sharaa, whose forces toppled longtime ruler Bashar al-Assad in late 2024, has vowed to bring all of Syria under state control - including SDF-held areas in the northeast.
But Kurdish authorities, who have run autonomous civilian and military institutions there for the last decade, have resisted joining up with Sharaa's Islamist-led government.
After a year-end deadline for the merger passed with little progress, Syrian troops launched an offensive in January.
They swiftly captured two key Arab-majority provinces from the SDF, bringing key oil fields, hydro-electric dams and some facilities holding Islamic State fighters and affiliated civilians under government control.
The US has been engaging in shuttle diplomacy to establish a lasting ceasefire and facilitate the integration of the SDF - once Washington's main partner in Syria - into the state led by its new US-favoured ally, Sharaa.
Senior officials from the United States and France, which has also been involved in talks, have urged Sharaa not to send his troops into remaining Kurdish-held areas, diplomatic sources told Reuters.
They fear that renewed fighting could lead to mass abuses against Kurdish civilians.
Government-affiliated forces killed almost 1500 people from the Alawite minority and hundreds of Druze people in sectarian violence in 2025, including in execution-style killings.
Amid the instability in the northeast, the US military has been transferring hundreds of detained fighters from the Islamic State group from Syrian prisons across the border into Iraq.
Iraq's Foreign Minister Fuad Hussein told the EU's foreign policy chief, Kaja Kallas, in a phone call on Saturday that Baghdad should not bear the "security and financial burdens" of the transfer of IS prisoners alone, the Iraqi foreign ministry said in a statement.
Turkey's Fidan, speaking on broadcaster NTV late on Friday, cited these transfers as possibly necessitating an extension to the Saturday deadline.
