It was the highest death toll for a single day in a week of fighting in the divided city of Aleppo between rebels and regime forces that has cost more than 200 lives, a monitoring group said.
UN envoy Staffan de Mistura warned that a hard-won February 27 ceasefire was now "barely alive", appealing for urgent action by its co-sponsors Russia and the United States to rescue it.
At the Security Council, UN aid chief Stephen O'Brien echoed de Mistura, urging world powers to revive the truce and put an end to the "massive human suffering" in war-torn Syria.
"Now is the time to launch the battle for the complete liberation of Aleppo," the paper said, adding that it "will not take long to begin, nor to finish".
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The regime source told AFP that "the army is preparing a huge operation in the coming days to push the rebels away from the city by encircling it and creating a security zone".
Rebels have controlled eastern districts of Aleppo city since 2012, while western neighbourhoods are held by the regime.
Further north in the province, rebels including the powerful Islamist Ahrar al-Sham group and Kurdish forces fought fierce battles yesterday and early today that left 64 fighters dead, a monitor said.
The upsurge in violence in and around Aleppo has strained the February truce between the government and non-jihadist rebels and cast a shadow over UN efforts to convene a new round of peace talks next month and channel aid to Syria's beleaguered population.
Regime air strikes on rebel-held districts killed 31 civilians, including three children, according to the British-based monitor.