Syrian opposition sources have reported extensive shelling by government forces of areas outside the capital, Damascus.
They say more than 100 people have been killed and witnesses claim poisonous gas has been used.
Graphic video, which has not been independently verified, shows images of bodies and children convulsing.
The BBC reports that footage uploaded to YouTube by activists shows many people being treated in makeshift hospitals.
A team of United Nations inspectors arrived in Syria on Sunday to investigate earlier allegations of chemical weapon use.
Syrian state news agency Sana said the reports of the attack were baseless.
