Fragile truce will be renewed every 48 hours and if it holds for a week, US and Russia will take on DAESH together.
A US and Russia-brokered ceasefire dubbed by Washington as perhaps the "last chance to save Syria" entered its first full day Tuesday, amid scepticism over how long it would hold.
An initial 48-hour truce came into force at sundown on Monday across Syria except in areas held by DAESH.
Fighting appeared to have stopped in Syria's devastated second city Aleppo, divided between the opposition-held east and regime-controlled west since mid-2012, as the ceasefire took effect.
Activists have reported at least 10 truce violations from areas such as Damascus and Idlib that have yet to be verified.
On Monday evening, the Syrian Observatory for Human Rights monitoring group said that it was "quiet" on nearly all fronts.
No martyrs recorded in the first 15 hours of the casefire agreement in Syria https://t.co/qRhUCAwrEA— # #SOHR (@syriahr) September 13, 2016
US 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 that was 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.
Opposition demand 'guarantees'
Syria's opposition groups have remained deeply sceptical and have yet to endorse the deal.
They sent a letter to Washington on Sunday saying they would "deal positively with the idea of the ceasefire" but listed several "concerns" and stopped short of a full endorsement.
"The clauses of the agreement that have been shared with us do not include any clear guarantees or monitoring mechanisms... or repercussions if there are truce violations," they said. Opposition groups said they had received no response.
Salem al-Muslet, spokesman for the High Negotiations Committee opposition umbrella group, demanded "guarantees" on which opposition groups would be targeted.
"We fear that Russia will classify all the Free Syrian Army as terrorists," as it was unclear how the deal defined "terrorist groups", he added.
A crucial part of the deal calls for opposition to distance themselves from the Fateh al-Sham Front — previously known as Al-Nusra Front — before joint US-Russian operations begin.
The Syrian regime and its allies Iran and Lebanon's Hezbollah movement have backed the truce.
Kerry said humanitarian assistance needed to urgently start flowing, including in all areas of Aleppo.
Under the deal, fighting will halt across areas not held by DAESH and aid deliveries to besieged areas will begin, with regime and rebel forces ensuring unimpeded humanitarian access to Aleppo in particular.
"I don't think I have to spell out how urgent this assistance is, in some cases literally the difference between life and death for tens of thousands of people," he added.
Kerry said the truce was designed to enable warring sides to resume negotiations on a political transition in Syria.
"This is designed to bring people to the table in Geneva," he added.
Kerry said once there was a sustained period of calm and increased humanitarian access over seven days, the United States and Russia would begin to coordinate military strikes against DAESH. The ceasefire will be renewed every 48 hours.
"As soon as US-Russia strikes begin then the Syrian regime will be prevented from flying combat missions over areas in which the legitimate opposition is present," 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.
'We aren't very hopeful'
Syria's regime forces announced a seven-day "freeze" on military operations, but opposition forces have yet to formally sign up to the truce.
The deal's fragility was underscored even before it took effect when Syrian regime leader Bashar al-Assad vowed to retake all of Syria from "terrorists" in a state televised interview.
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.
Senior Russian military official Sergei Rudskoi said the "cessation of hostilities is being resumed across all the territory of Syria" but that Moscow would "continue to carry out strikes against terrorist targets".
Russian Deputy Foreign Minister Mikhail Bogdanov said UN special envoy Staffan de Mistura could invite parties to new peace talks "at the very beginning of October".
Kerry also said that the only "realistic and possible solution" to the conflict would be a return to UN-mediated peace talks.
Just before the truce began, the UK-based Syrian Observatory for Human Rights reported 13 people were killed by regime shelling of the rebel town of Douma near Damascus and another 13 killed in unidentified raids in Idlib province.
13 killed in the first day of Eid around Syria https://t.co/zb9KwU1rVF— # #SOHR (@syriahr) September 12, 2016
Bombardment rocked the central town of Talbisseh all day, an activist there said, finally quieting down as the truce came into effect.
"We spent Eid in our bomb shelters and basements today," said Hassan Abu Nuh of the Muslim holiday.