The air, rocket and artillery campaign is the heaviest since rebels overran a helicopter base and missile base near Damascus two months ago and encroached on the main international airport, inching steadily closer to the capital, the sources said.
Video footage showed women weeping over the dismembered bodies of children strewn across a field in Eastern Ghouta, near an air defence base on the edge of the town of Muleiha, three miles east of Damascus.
Muleiha is the last major fortification in the area east of Damascus not yet to have fallen to the rebels. President Bashar al Assad's core troops and security personnel are entrenched in the capital.
"God is greater than you, Bashar," one of the activists is shown saying as a youth carries the torso of a child.
The footage was taken by activists and could not be independently verified.