A cessation of hostilities is due to start in Syria at sunset on Monday, after a weekend of air strikes.
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 had 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 major rebel group, the hardline Islamist Ahrar al-Sham, has rejected the deal, which is scheduled to come into effect around 15:45 GMT.
“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.
And President Assad, speaking hours before the ceasefire was due to start, said the Syrian state was still “determined to recover every area from the terrorists, and to rebuild”.
Even before the ceasefire comes into effect there are signs of problems with the plan.
Armed opposition groups have formed alliances to fight the Syrian army. They range from groups supported by the Americans and their allies to ones that have had links with al-Qaeda.
The ceasefire calls on US backed groups to separate from the others. They’re reluctant to do so – one saying they’re in the same trench, with joint operations rooms.
That is just one part of a complicated agreement that might crack open when it comes up against the realities of the Syrian war.
Ahead of the ceasefire, the Syrian government carried out heavy air strikes in several rebel areas over the weekend, killing about 100 people. -BBC