Dozens of people were killed or wounded in fierce army bombardment of areas near Damascus on Wednesday, an NGO said, in what activists charged was a toxic gas attack.
The allegation of chemical weapons being used in the heavily-populated areas came on the second day of a mission to Syria by UN inspectors. It was promptly denied by the Syrian authorities.
"Regime forces ... stepped up military operations in the Eastern Ghouta and Western Ghouta zones of the Damascus region with aircraft and rocket launchers, causing several dozen dead and wounded," said the Syrian Observatory for Human Rights.
The intensive bombing on the outskirts of the capital could be heard by residents of Damascus, where a grey cloud capped the sky.
The Britain-based Observatory, which relies on a vast network of activists on the ground and medics, said the army operation was aimed at the recapture of Madhamiyat el-Sham, an area southwest of Damascus.
The Local Coordination Committees (LCC), a network of activists reported hundreds of casualties in the "brutal use of toxic gas by the criminal regime in parts of Western Ghouta".
And in videos posted on YouTube, the Syrian Revolution General Commission, another activist group, showed what it called "a terrible massacre committed by regime forces with toxic gas, leaving dozens of martyrs and wounded."
The attack "led to suffocation of the children and overcrowding field hospitals with hundreds of casualties amid extreme shortage of medical supplies to rescue the victims, particularly Atropine," the LCC said in an English-language statement.
Eastern Ghouta "was also shelled by warplanes following the chemical attack that is still ongoing which led to hundreds of casualties and victims, among them entire families," it said.
In one video, children are seen being given first aid in a field hospital, notably oxygen to help them breathe. Doctors appear to be trying to resuscitate unconscious children who show no signs of bleeding.
The authenticity of the videos could not immediately be verified.
The Syrian Observatory called for inspectors to hastily visit the stricken sites and ensure access for medical aid.
UN chief Ban Ki-moon insisted Monday that the inspectors be granted unrestricted access to Syrian sites where chemical weapons have allegedly been used in the country's 29-month-old conflict.
The inspectors, expected to visit three sites including Khan al-Assal near Aleppo in the north, are due to be in Syria for 14 days, with the possibility for an extension of the mission.
"In order to credibly establish the facts, the mission must have full access to the sites of the alleged incidents," the secretary general told reporters.