A cessation of hostilities has come into effect in Syria, although it is unclear how widely it will be observed.
If the truce holds for seven days, the US and Russia will carry out co-ordinated air strikes against jihadist militants.
The Syrian army says it is observing the truce but it is still unclear whether rebels will abide by it.
The cessation of violence followed a weekend of air strikes by government forces on several rebel areas.
After the ceasefire came into effect at sunset on Monday, the Syrian army announced a seven-day "freeze" on military operations.
The Syrian Observatory for Human Rights monitoring group reported that calm appeared to be prevailing on most front lines.
The deal was struck on Friday in Geneva after months of talks between Russia and the US.
The opposition Free Syrian Army group has said that while it will "co-operate positively" with the ceasefire, it was concerned it would benefit the government.
Another major rebel group, the hardline Islamist Ahrar al-Sham, initially rejected the deal but later appeared to have softened its stance.
Opposition sources quoted by Reuters said that a forthcoming statement supporting the cessation "with harsh reservations" would be backed by "the largest groups", including Ahrar al-Sham.
Speaking earlier, President Bashar al-Assad welcomed the deal but said the Syrian state was still "determined to recover every area from the terrorists, and to rebuild".
'Civilians have no hope any more'
The cessation of violence is due to be renewed every 48 hours.
Over the weekend, the Syrian government carried out heavy air strikes in several rebel areas, killing about 100 people.
Russian warplanes were also in action in the provinces of Idlib and Aleppo, say Syrian activists.
Such intensification of violence has occurred before other, aborted, ceasefires in Syria.
"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 any more."
Under the plan, Syrian government forces will halt 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.
More than 4.8 million have fled abroad, and an estimated 6.5 million others have been displaced within the country, the UN says.