Insurgents now control the vast majority of Idlib after Al-Nusra Front -- Al-Qaeda's Syrian affiliate -- and its allies overran the last remaining regime-held city and surrounding villages.
The Syrian Observatory for Human Rights said "at least 71 civilians were killed, and dozens were wounded, when regime helicopters dropped barrel bombs on the city of Al-Bab and in Al-Shaar in east Aleppo city".
Rami Abdel Rahman, the director of the Britain-based monitoring group, said 12 people were killed in rebel-held Al-Shaar, including eight members of a single family.
Bulldozers were used to clear away the rubble by civil defence volunteers.
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One of them, Shahud Hussein, said the blasts were so powerful that buildings in the neighbourhood were "likely to collapse".
The other 59 civilians, all male, were killed at a market in Al-Bab, Abdel Rahman told AFP.
Al-Bab lies about 40 kilometres northeast of Aleppo city and is controlled by the extremist Islamic State group.
Those killed were all male because women have much less freedom of movement in IS-controlled areas, he added.
Barrel bombs are crude weapons made of oil drums, gas cylinders or water tanks packed with explosives and scrap metal that are usually dropped from helicopters.
The Syrian government's use of the weapons has come under fire by rights groups, who say they are indiscriminate and often kill many civilians.
The Observatory said regime forces also dropped barrel bombs yesterday in Idlib province, now under the de facto control of rebels after the Army of Conquest opposition alliance captured the city of Ariha and surrounding villages.