The 10-day truce is due to be followed by co-ordinated US-Russian air strikes against jihadist militants.
Syrian state media reported that President Bashar al-Assad welcomed the deal, which was reached late on Friday in Geneva after months of talks between Russia and the US.
But it is unclear whether rebel factions will abide by it.
The Free Syrian Army group has written to the United States administration saying that while it would "co-operate positively" with the ceasefire, it was concerned it would benefit the government.
Another rebel group, the influential hardline Islamist Ahrar al-Sham, has rejected deal.
"A rebellious people who have fought and suffered for six years cannot accept half-solutions," said its second-in-command, Ali al-Omar, in a video statement. But the group`s commander stopped short of explicitly saying it would not abide by its terms.
War in Syria: Cessation of hostilities due to start at sunset
12 September 2016
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A cessation of hostilities is due to start in Syria at sunset on Monday, after a weekend of air strikes.
