While it is not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances, experts say it would be almost impossible to fake so many dead and injured including children and babies.
Both the rebels and government forces have accused each other of using chemical weapons throughout the 28-month conflict.
Syria is believed to have large undeclared stockpiles of mustard gas and sarin nerve agent.
The government has implied it has chemical weapons, but said they would not be used against civilians.
More than 100,000 people are believed to have been killed during the 28-months of conflict Syria.