United Nations chief Ban Ki-moon has called for an investigation over Wednesday’s alleged gas attack in the suburbs of Damascus.
Distressing footage has been shown of what the Syrian opposition claims are the after-effects of rocket fire, which released deadly fumes over rebel-held areas.
Up to 1,300 people are said to have been killed.
The Syrian government has denied any involvement and accused the rebels of a propaganda ploy.
Thirty-five member countries have called for the UN inspection team already there to be allowed to investigate quickly.
UN Deputy Secretary-General Jan Eliasson told reporters: “This represents, no matter what the consequences are, a serious escalation with grave humanitarian consequences and human consequences. We very much hope that we will be able to conduct the investigation. Dr. Sellstrom and his team are in place in Damascus. We hope that they will be given access to the area by the government.”
As the UN Security Council met in emergency session over the incident, protesters gathered near its headquarters.
President Barack Obama has made the use of chemical weapons a “red line” that in June triggered more US aid to the rebels, but not the all-out military intervention their leaders have sought.