Air strikes on Syria's battleground city of Aleppo killed six civilians on Sunday just 24 hours before a truce brokered by Russia and the United States was due to begin.
The ceasefire, announced after marathon talks by the Russian and US foreign ministers, has been billed as the best chance yet to end Syria's five-year civil war estimated to have killed more than 290,000 people.
As the clock ticked towards sunset on Monday when the ceasefire is expected to start, rebels battling the Syrian regime and the political opposition were still weighing whether to take part in the truce.
Only one rebel group, Ahrar al-Sham, had given its official reaction -- a rejection of the truce deal, which the influential hardline Islamists said would only serve to strengthen the regime of President Bashar al-Assad.
Key regime ally Iran welcomed the plan on Sunday and called for "comprehensive monitoring" of the truce, particularly along Syria's volatile borders.
But even as world powers threw their support behind the deal, regime air raids on rebel-held parts of northern city Aleppo killed six civilians and wounded 30, the Syrian Observatory for Human Rights said.
The latest bloodshed comes a day after air raids by unidentified warplanes hit two key northern cities in opposition-held territory.
At least 62 people, including 13 women and 13 children, were killed in Saturday's bombardment of Idlib city, the Britain-based Observatory said.
The strikes hit several areas including a market full of shoppers preparing for the Muslim holiday of Eid al-Adha, which begins on Monday.
Britain's special representative for Syria, Gareth Bayley, called the attacks "barbaric".
"Bring on the #SyriaCeasefire," he tweeted.
Aleppo was hit for the second day on Sunday, the Observatory said, after 12 civilians were killed there in air strikes on Saturday.
"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."
In a major blow to the opposition, pro-regime forces reimposed a devastating siege on Aleppo's eastern districts last week.
- Assad, allies welcome truce -
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State news agency SANA reported Saturday that Syrian President Bashar al-Assad's government "approved the agreement" for a truce.
Lebanese Shiite militia Hezbollah, which has intervened militarily on behalf of Assad, also announced its support.
Key Assad and Hezbollah backer Iran also welcomed the deal, although foreign ministry spokesman Bahram Ghasemi cautioned that its success relies on creating "a comprehensive monitoring mechanism, in particular control of borders in order to stop the dispatch of fresh terrorists" to Syria.
Syria's main opposition group the High Negotiations Committee -- grouping political dissidents as well as armed rebel factions -- had yet to formally respond.
But the hardline Ahrar al-Sham, which works closely with former Al-Qaeda affiliate Fateh al-Sham Front, rejected the deal, saying it would "send all the sacrifices and gains of our people who have risen up into smoke".
Fateh al-Sham is not covered by the truce agreement but it too dismissed the plan, with spokesman Mostafa Mahamed writing on Twitter: "Negotiations and deals which do not take account of fighters on the ground are useless."
- Syrians 'have lost faith' -
In the capital Damascus, resident Taher Ibrahim said he did not expect any lasting respite from the fighting.
"Nobody among the Syrian population accepts this agreement... (the opposition) are all the same and none of them will commit to this truce," he said.
But in rebel-held Douma, besieged by government forces since 2013, the local council said it backed the truce and appealed for peace, stressing: "Enough of war."
The agreement was reached after talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in Geneva.
It would see fighting and indiscriminate air attacks halt across the country, starting at sundown on Monday for 48 hours, which could then be renewed.
To get aid into Aleppo, a "demilitarised zone" would be established around the Castello Road into the city.
If the ceasefire holds for one week, the US and Russia could start joint operations against jihadists from the Islamic State group and Fateh al-Sham.
Pro-government Syrian newspaper Al-Watan said on Sunday the deal would pave the way for renewed peace talks in Geneva.
Several attempts at negotiations have failed since the conflict erupted, with talks earlier this year in the Swiss city fizzling out after the opposition walked away in protest at the humanitarian situation.