At least 42 people, including children, were killed on Sunday when Syrian army helicopters dropped improvised "barrel bombs" in the northern province of Aleppo, a monitoring group said.
The British-based Syrian Observatory for Human Rights said separate air raids hit several districts in Aleppo, but the biggest toll was in Hanano, east of the city. At least six children were among the dead.
"They hit a convoy of cars on a road in Hanano, many cars were destroyed. There were civilians there," said the Observatory's Rami Abdelrahman.
Human Rights Watch said in a report over the weekend that barrel bomb attacks had killed scores of civilians in Aleppo in the last month. It described the attacks as illegal and said they had hit residential and shopping areas.
"The Syrian air force is either criminally incompetent, doesn't care whether it kills scores of civilians, or deliberately targets civilian areas," HRW senior emergency researcher Ole Solvang said in the report.
Barrel bombs are explosive-filled cylinders or oil drums that are often rolled out of the back of helicopters with little attempt at striking a particular target. They are capable of causing widespread casualties and significant damage.
President Bashar al-Assad's forces, battling rebels in a 2 1/2-year conflict that has killed more than 100,000 people, frequently deploy air power and artillery against rebel-held districts across the country.
They have been unable to recapture eastern and central parts of Aleppo, which rebels seized in the summer of 2012, but they have driven rebel fighters back from towns to the southeast of the city in recent weeks.