The Syrian Observatory for Human Rights, a Britain-based monitor, said that fighting and rocket fire had stopped in the rebel bastion of Zabadani and the besieged government-held villages of Fuaa and Kafraya.
Earlier, Mohammed Abu Qassem, secretary general of Syria's Tadamun (Solidarity) Party and a mediator of the truce, confirmed the ceasefire was due to begin at 6:00 am local time.
The new two-day truce follows a similar ceasefire earlier this month for the towns that was intended to lead to a broad agreement to end the fighting in Zabadani and the blockade of Fuaa and Kafraya.
Pro-regime forces, including Lebanon's Shiite Hezbollah militia, launched an offensive to seize Zabadani from rebel groups early last month.
The town is the last rebel-held bastion in the area along the border with Lebanon and has been subjected to massive aerial bombardment since the operation began.
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In retaliation, a rebel alliance including Al-Qaeda affiliate Al-Nusra Front surrounded Fuaa and Kafraya, two Shiite Muslim villages in Idlib province, and began firing rockets into them.
The first ceasefire for the three towns began on August 12 as a 48-hour truce but was extended for another day as negotiators tried but ultimately failed to reach a broad deal.
The earlier talks included discussion of the withdrawal of rebels from Zabadani, perhaps in exchange for the evacuation of civilians from Fuaa and Kafraya or the delivery of aid.
The rebels have also sought the release of prisoners held by the regime.
Negotiations on those points were now ongoing, Observatory director Rami Abdel Rahman said.
Fuaa and Kafraya are the only remaining government-held villages in Idlib province in northwest Syria.
The regime still holds a military airport there, but was otherwise expelled by a rebel alliance earlier this year.