Russia said it was continuing to bomb “terrorists” in parts of Syria, BBC reports.
Almost 100 rebel factions have agreed to respect the truce, the main Syrian opposition group has said.
The High Negotiations Committee (HNC) said Free Syrian Army factions and the armed opposition had signed up to the truce from midnight (22:00 GMT).
The temporary “cessation of hostilities” involves government and rebel forces – but not the so-called Islamic State (IS) group and the al Qaeda-linked Nusra Front. On Friday, Nusra Front urged its supporters to intensify attacks against President Bashar al-Assad and his allies.
Warring parties in Syria were meant to make their intentions known by midday Friday ahead of the pause in fighting.
Announcing the intentions of rebel factions, the HNC said the Syrian government and its allies must not use the “proposed text to continue the hostile operations against the opposition factions under the excuse of fighting terrorism”.