An internationally brokered ceasefire in Syria was due to begin at sundown on Monday but, with only hours to go, the country's opposition forces had yet to sign on.
In a further sign of the deal's fragility, Syria's President Bashar al-Assad vowed to retake the whole country from "terrorists".
The deal, announced Friday after marathon talks between Russia and the United States, has been billed as the best chance yet to halt the bloodshed in Syria's five-year civil war.
As well as bringing a temporary end to the fighting, it aims to provide crucial aid to hundreds of thousands of desperate civilians.
Under the agreement, an initial 48-hour ceasefire is to begin at 7:00 pm local time (1600 GMT), halting fighting in areas not held by jihadists like the Islamic State group.
Aid deliveries to many besieged and "hard-to-reach" areas are set to simultaneously begin, with government and rebel forces ensuring unimpeded humanitarian access in particular to divided Aleppo city.
The ceasefire will be renewed every 48 hours and, if it holds for a week, Moscow and Washington will begin unprecedented joint targeting of jihadist forces.
After years of stalled peace efforts and the failure of a landmark truce agreed in February, world powers are anxious to end a conflict that has killed more than 290,000 people.
Russia's deputy foreign minister Mikhail Bogdanov told state-run news agency RIA Novosti that peace talks could resume within a month.
"I think that probably at the very beginning of October, (UN envoy Staffan) de Mistura should be inviting all parties" to talks, he said.
- Rebels seek US assurances -
But Syria's opposition and rebels are deeply sceptical that Assad's regime will abide by the truce agreement, and demanded guarantees before endorsing a deal.
"We are asking for guarantees especially from the United States, which is a party to the agreement," Salem al-Muslet from the High Negotiations Committee, the main opposition umbrella group, told AFP Monday.
"We fear that Russia will classify all the Free Syrian Army (rebel factions) as terrorists," as it was unclear how the deal defined "terrorist groups", he said.
Rebel groups on Sunday sent a letter to the US 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.
Signup to our newsletter and follow us on Facebook and Twitter!
Ahmad al-Saoud, who heads the US-backed Division 13 rebel group which signed the letter, said they had received no response.
Questions also remain about how the ceasefire will apply in parts of Syria where the Fateh al-Sham Front, previously known as Al-Nusra Front, is present.
A crucial part of the deal calls for rebels to distance themselves from the group before joint US-Russian operations against it begin.
But Fateh al-Sham cooperates closely with many of Syria's rebels, including the powerful Ahrar al-Sham faction, which on Sunday issued a scathing condemnation of the Russian-US deal.
In a message marking the Muslim holiday of Eid al-Adha, Ahrar al-Sham's deputy leader Ali al-Omar said Syria's "people cannot accept half-solutions".
His remarks suggested a rejection of the deal but hours later Ahrar al-Sham spokesman Ahmed Qara Ali told AFP that they were meant to note the deal's "drawbacks" ahead of a future formal reply.
- Assad vows to retake all Syria -
Ahrar al-Sham is Syria's most powerful non-jihadist rebel group, with a commanding presence in Aleppo and Idlib province, which it rules as part of the Army of Conquest alliance with Fateh al-Sham.
Syria's government and its allies including Iran and Lebanon's Hezbollah movement have backed the truce.
But on Monday Assad made clear he was intent on recapturing all of Syria.
"The Syrian state is determined to recover every area from the terrorists," he said as he toured Daraya, a former rebel stronghold that surrendered last month after a four-year government siege.
"The armed forces are continuing their work, relentlessly and without hesitation, regardless of internal or external circumstances," he added.
The run-up to the truce has also seen a spike in violence, with at least 74 people killed in strikes on Aleppo and Idlib cities over the weekend.
Fresh raids hit Aleppo Monday, an AFP correspondent said, where residents have struggled to celebrate Eid amid shortages created by a renewed government siege.
Aleppo has been divided between rebels in the east and regime forces in the west since mid-2012.
In August, rebels broke a weeks-long regime siege of the east, but Assad loyalists restored the blockade on September 8.
"We hope there will be a ceasefire so that civilians can get a break," said east Aleppo resident Abu Abdullah. "Civilians have no hope any more."