The Free Syrian Army groups wrote to the United States on Sunday about the deal it agreed on with Russia, saying that while they would "cooperate positively" with the ceasefire, they were concerned it would benefit the government.
Although the letter did not explicitly say the groups would abide by the ceasefire, two rebels who confirmed its text to Reuters said they would respect the ceasefire when it comes into force on Monday evening.
But according to the letter, the groups are worried by the absence of enforcement mechanisms, a lack of provision for besieged areas and clauses letting army jets fly for up to nine days after the deal comes into effect.
The influential hardline Islamist rebel group Ahrar al-Sham issued a statement late on Sunday attacking the ceasefire deal, but stopping short of explicitly saying it would not abide by its terms.
A war monitor reported clashes around Aleppo and Damascus, but pushes by the government in the mountainous north-west and rebels in the south-west indicated an effort to improve their positions before fighting is due to stop on Monday.
Syria's five-year civil war has killed hundreds of thousands of people and displaced 11 million - half the country's prewar population - causing a refugee crisis in the Middle East and Europe and inspiring jihadist attacks around the world.
Underscoring the war's global impact, President Bashar al-Assad is backed by Russia's air force, Iran's Revolutionary Guards and Shi'ite militias from Iraq and Lebanon, while the rebels are supported by the United States, Turkey and Gulf Arab states.