At least 91 people, including 17 children, were killed on Friday in attacks on opposition strongholds in the north and outside Damascus, the Syrian Observatory for Human Rights said.
The deaths came as top diplomats from 17 countries, including Iran and Saudi Arabia, met for the first time in Vienna hoping to find a political solution to the conflict.
Of the 91, at least 59 died in government attacks on Douma, a town on the eastern edges of the capital.
"The toll has risen to 57 people, including five children and two women" when more than a dozen regime rockets struck the town, the monitoring group said.
Another two people were killed in government air raids on Duma later Friday, Observatory head Rami Abdel Rahman said.
Douma is in Eastern Ghouta, the largest opposition stronghold in Damascus province.
Elsewhere, 32 civilians, among them 12 children, were killed on Friday afternoon in air strikes on opposition-held areas of Syria's second city Aleppo, the Observatory said.
The Britain-based monitor said the strikes were believed to have been carried out by regime or Russian warplanes.
It said 10 were killed in the Fardous neighbourhood, and two children were killed in the Salaheddin district.
The raids killed another 20 civilians -- half of them children -- in Maghayir, the Observatory reported, adding that dozens were wounded or missing.
More than 250,000 people have been killed since Syria's war began in March 2011.
Both the government and opposition forces have been condemned by rights groups for firing indiscriminately on civilian areas.
The opposition National Coalition said those killed in Douma on Friday morning had been struck by Russian air raids.
"The National Coalition holds Russia, Iran, and the regime of (President) Bashar al-Assad responsible for the deaths of these civilians," a coalition official told AFP, speaking on condition of anonymity.
- Frequent target -
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He said the attacks "indicated an attempt to undermine the efforts to stop the violence" during the Vienna meeting.
Douma has been a frequent target of government attacks.
"Douma is one of the areas in Syria where there are the highest number of deaths since the beginning of the war," Abdel Rahman said.
Government forces regularly target it with rocket fire, shelling and air raids, and opposition groups in Douma also launch rockets into the capital.
In August, 117 people were killed in a single day of air strikes in the town, causing a global outcry.
Government air strikes there on Thursday hit a market and a hospital, killing at least nine people, the Observatory said.
An AFP photographer said Thursday's attack had wounded hospital staff, limiting the treatment available for Friday's wounded.
He said the latest attack as residents gathered at the market left corpses piled on top of each other.
In the chaotic aftermath, a man in his thirties cried over the body of a young boy.
"Since your father was killed in the last massacre, your mother has been telling you to stop working in the market. Why did you go? Why?" he cried.
Corrugated metal rooftops, twisted and blown apart in the attack, were left dangling over mangled bicycles and shredded signs.
The Douma Coordination Committee, a local activist group, published a gruesome video of what it said was the aftermath of more than a dozen rockets hitting the market.
Blood-soaked bodies lay crumpled underneath tables, as a young boy in a sky-blue sweater stood on the sidelines, looking stunned.
In Vienna, major powers narrowed their differences on Syria but remained divided over the key issue of Assad's future, France's foreign minister said.
Another round of talks will be held in two weeks' time, Laurent Fabius told reporters.