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 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.
Ahead of the ceasefire, the Syrian government carried out heavy air strikes in several rebel areas over the weekend, killing about 100 people.
Russian warplanes have also been in action in the provinces of Idlib and Aleppo, say Syrian activists.
Such intensification of violence has occurred before other, aborted, ceasefires in Syria, reports the BBC’s Middle East analyst Sebastian Usher.
“We hope there will be a ceasefire so that civilians can get a break. The shelling goes on night and day, there are targeted killings, besieged cities,” said Abu Abdullah, who lives in Aleppo’s rebel-held east.
“Civilians have no hope anymore,” he added.
Under the plan, Syrian government forces will end combat missions in specified opposition-held areas.
Russia and the US will then establish a joint centre to combat jihadist groups, including so-called Islamic State and Jabhat Fateh al-Sham (known until recently as the Nusra Front).
The conflict in Syria, which began with an uprising against Mr Assad, has raged for five years and claimed the lives of more than a quarter of a million people.
Millions have fled abroad, many of them seeking asylum in the EU, but nearly 18 million people remain in Syria, which has been carved up by fighting between government and rebel forces.
Jihadist groups like so-called Islamic State and Jabhat Fateh al-Sham (formerly known as Nusra Front) face the joint might of the Russian and US air forces
Moderate rebels and civilians in the areas they hold will no longer face the threat of indiscriminate air strikes such as barrel-bombing although the Syrian air force will not be grounded completely; aid deliveries will be allowed to areas currently under siege
President Assad will be in a stronger position as the US and Russia engage two of his most effective military opponents while moderate rebels observe the truce with his forces