The attack coincided with the visit by a 20-member UN team to Syria to investigate three sites where chemical weapons attacks allegedly occurred during the past year.
The reported death toll would make it the deadliest alleged chemical attack in Syria's civil war. There have been conflicting reports, however, as to what exactly took place.
This picture shows a youth who survived the alleged gas attack, crying as he takes shelter inside a mosque in the Duma neighbourhood of Damascus.
Click through for more images, which contain scenes that some may find disturbing