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.
Key regime ally Iran welcomed the plan on Sunday and called for "comprehensive monitoring" of the truce, particularly along Syria's volatile borders.
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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 on Sunday.
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.
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.
Iran, a key backer of Assad and Hezbollah, also welcomed the truce deal, foreign ministry spokesman Bahram Ghasemi said.