A U.S.- and Russia-brokered ceasefire dubbed by Washington as maybe the "last chance to save Syria" took effect at sundown on Monday, amid skepticism over how long it would hold.
An initial 48-hour truce came into force at 7:00 p.m. local time (1600 GMT) across Syria except in areas held by jihadists such as the Islamic State (ISIS) group.
AFP correspondents in Syria's devastated second city Aleppo, divided between the rebel-held east and regime-controlled west since mid-2012, said fighting appeared to have stopped as the ceasefire took effect.
A final rocket was fired from the east into government areas just five minutes before 7:00 p.m.
The Syrian Observatory for Human Rights monitoring group said later it was "quiet" on nearly all fronts.
Secretary of State John Kerry told reporters in Washington it was "far too early to draw conclusions", but noted that reports he received two hours after the truce came into effect suggested "some reduction" in violence.
"For all the doubts that remain, and there will be challenges in the days to come, this plan has a chance to work," he said of the deal he agreed on Friday with his Russian counterpart Sergei Lavrov.
"And I urge all the parties to support it because it may be the last chance that one has to save a united Syria," Kerry said.
On the ground in Syria, residents hoped for the best.
"I was checking the time all day, waiting for it to turn 7:00," said Khaled al-Muraweh, a 38-year-old shopkeeper in western Aleppo's Furqan district.
"I hope the ceasefire holds so I can see my brother who lives in the opposition-held part of the city."
In the east, residents celebrated the start of the Muslim holiday of Eid al-Adha.
"This was the calmest day since I got married a week ago," said Shadi Saber, 26.
Syria's armed forces announced a seven-day "freeze" on military operations, but opposition forces have yet to formally sign up to the truce.
In fact, hours before the ceasefire was to begin, Syria's main opposition umbrella group and several rebel factions called for "guarantees" on the implementation of a truce deal before endorsing it.
The deal's fragility was underscored even before it took effect when President Bashar Al-Assad vowed to retake all of Syria from "terrorists", the term his regime uses to describe all rebel groups seeking his ouster.
It is the latest in a series of efforts to try to halt Syria's five-year war that has killed more than 290,000 people and displaced half the population.
Under the deal, fighting will halt across areas not held by jihadists and aid deliveries to besieged areas will begin, with government and rebel forces ensuring unimpeded humanitarian access to Aleppo in particular.
Rebels broke a regime siege of the east in August, but Assad loyalists restored the blockade on September 8.
The ceasefire will be renewed every 48 hours and, if it holds for a week, Moscow and Washington will begin an unprecedented joint campaign to target jihadist forces.
AFP contributed to this report.