Sources close to negotiations on both sides earlier told Reuters agreement had been reached on a new ceasefire in the town of Zabadani and the two villages of Kefraya and al Foua in the northwestern province of Idlib.
"Implementation has started. There is calm in Kefraya, al Foua and Zabadani," Rami Abdulrahman, who runs the Observatory, told Reuters.
The negotiations have been led on the insurgent side by Ahrar al-Sham, a conservative Sunni Islamist insurgent group. The ceasefires are in the western portion of Syria, beyond the main strongholds of Islamic State fighters.
A similar ceasefire this month in the same areas, organized with help from Iran and Turkey, broke down after a brief extension, and fighting has raged since.
A fighter on the side of government forces in Zabadani said clashes had intensified prior to the latest planned ceasefire.
The previous ceasefire, which began Aug. 12, was intended to give a chance for negotiations aimed at a more lasting cessation of hostilities in both areas.
Iran and Turkey back the Syrian government and insurgents, respectively.
That ceasefire also aimed to secure the withdrawal of rebel fighters from Zabadani and of citizens from the two villages.
Ahrar al-Sham blamed the failure of that round of talks on the Iranian delegation with which it was negotiating, saying it was trying to effectively exchange one area for another.
Zabadani has been the focus of an offensive by Hezbollah and the Syrian army against insurgent groups holed up there. The area is of crucial importance to President Bashar al-Assad because of its proximity to Damascus and the Lebanese border.
Insurgent groups have in turn launched attacks on the two Shi`ite villages in the northwestern province of Idlib, an area bordering Turkey that is mostly insurgent-controlled after a series of advances against the army this year.
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