The main umbrella organisation for Syrian opposition groups backed by the west and Saudi Arabia has said armed groups fighting President Bashar al-Assad will respect a two-week truce beginning at midnight local time (10pm GMT).
The high negotiations committee (HNC) said nearly 100 rebel factions had agreed to the ceasefire, adding that the Syrian government and its allies must not launch attacks on the pretext of fighting terrorism.
The Russian president, Vladimir Putin, welcomed the announcement, saying that though the planned ceasefire was complex, there was no alternative.
Under the terms of the deal, armed groups had to confirm their commitment to the US or Russia no later than midday Damascus time. It was not immediately clear how many factions had refused to join the ceasefire or their military significance.
Fighting was continuing across much of western Syria on Friday morning, with heavy airstrikes reported on rebel-held areas to the east of Damascus.