"A ceasefire in Zabadani and the villages of Fuaa and Kafraya was agreed under the sponsorship of the UN," Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP.
A UN spokeswoman, Jessy Chahine, told AFP: "We can confirm that there were positive developments in the talks, which were facilitated by the UN," but added that it was up to the parties themselves to say if there was a deal to announce.
Pro-government forces launched an offensive to try to recapture Zabadani on the border with Lebanon in July.
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This prompted a rebel alliance, including Sunni Muslim extremists of Al-Qaeda, to besiege the Idlib province villages of Fuaa and Kafraya, whose residents are Shiites.
"The agreement for a six-month truce was reached between rebel fighters including Islamist group Ahrar al-Sham and pro-regime forces and their allies of the Lebanese Shiite Hezbollah movement," Abdel Rahman said.
Short truces agreed previously between the warring parties were often broken.
A Syrian source close to the talks told AFP that after a ceasefire began last Sunday, the accord will see the evacuation of some 10,000 civilians from besieged Fuaa and Kafraya on Saturday and Sunday.
They will be driven out in Red Cross vehicles to regime-controlled areas. In exchange, some 500 rebels will withdraw from Zabadani and head for Idlib province.