More than 300 people were killed, among them 87 children, in eight days of consecutive Syrian government air strikes on rebel-held areas of the northern city of Aleppo, a Britain-based monitoring group said on Monday, dpa reported.
The group, the pro-opposition Syrian Observatory for Human Rights, said the casualties occurred between December 15-22.
In some of the strikes, the planes dropped barrels packed with explosives on rebel-held areas.
Aleppo, Syria's commercial hub, has long been at the centre of fighting between the Syrian military and rebels battling to oust President Bashar al-Assad.
Human Rights Watch accused government forces on Saturday of using weapons that did not distinguish between civilians and combatants.
The main Syrian opposition, the National Coalition, has called on Western countries to impose a no-fly zone on rebel-held areas to stop attacks by Syrian regime forces.
The Syrian crisis started in March 2011 with pro-democracy protests; since then more than 130,000 people have been killed in the conflict, according to rights groups.