Thirty-six people, nearly half of them children, were killed on Sunday when Syrian army helicopters dropped improvised "barrel bombs" on the disputed northern city of Aleppo, the Syrian Observatory for Human Rights said.
Video uploaded by local activists showed a fire in a narrow street covered in debris and dust after one air raid in the Karam el-Beik district. Another showed blackened and twisted wreckage of a vehicle at a busy roundabout.
The Britain-based Observatory said at least 15 of the casualties on Sunday were children.
Barrel bombs are explosive-filled cylinders or oil barrels, often rolled out of the back of helicopters with little attempt at striking a particular target but 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 stormed in the summer of 2012, but they have driven rebel fighters back from towns to the southeast of the city in recent weeks.
Backed by Lebanese Hezbollah guerrillas and Iraqi Shi'ite fighters, they have also consolidated Assad's control over the capital Damascus and the main highway north to the central city of Homs, despite counter attacks by the Sunni rebels, who include many foreign jihadi fighters.
The Observatory also said on Sunday the death toll from a sectarian attack by Islamist rebels on Wednesday in the town of Adra, northeast of Damascus, had risen to 28.
It said the dead included Alawites - the same minority sect which Assad belongs to - Druze and Shi'ite Muslims.