Syria has been accused of using so-called indiscriminate "barrel bombs" in an attack that is thought to have left 83 people dead.
Pictures of the attack - which cannot be independently verified - appear to show huge bombs dropping and exploding on the northern city of Aleppo and a suburb of the capital Damascus.
Most of the victims killed since Friday have been civilians from the city's eastern districts, including women and children, according to the British-based Syrian Observatory for Human Rights, which has a broad network of sources across Syria.
The use of barrel bombs - oil drums or cylinders packed with explosives and metal fragments - has drawn international condemnation, including from Syria's opposition delegation and their Western backers at recent peace talks in Switzerland.
Harry Smith reports: