Syria chemical attack, Rockets with toxic agents were launched at the suburbs of the Ghouta region early on Wednesday as part of a major bombardment on rebel forces, they say.
The Syrian army says the accusations have been fabricated to cover up rebel losses.
The main opposition alliance said that more than 1,000 people were killed by the attacks.
The United Nations Security Council said it was necessary to clarify what happened in the alleged attack, but stopped short of demanding an investigation by a UN team currently in Damascus, following an emergency meeting on Wednesday evening.
"There is a strong concern among council members about the allegations and a general sense that there must be clarity on what happened and the situation must be followed closely," Argentina's UN Ambassador Maria Cristina Perceval told reporters after a closed-door meeting.
Meanwhile, the US, UK and France are among some 35 member states that have signed a letter calling for the UN inspectors that are already investigating three sites of alleged chemical weapons use in Syria to probe the latest incident as soon as possible.
Activist networks reported death tolls from the incident in the hundreds, but these could not be independently confirmed.
It is also not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances.
Dozens of bodies with no visible signs of injuries, including small children, laid out on the floor of a clinic.Ghazwan Bwidany, a doctor treating the injured, told the BBC the main symptom, especially among children, was suffocation, as well as salivating and blurred vision.
"We don't have the capability to treat all this number of people," he said.
"We're putting them in mosques, in schools. We are lacking medical supplies now, especially atropine, which is the antidote for chemical weapons."
In a statement, the army described the accusations of chemical weapons use as grave, and stressed the military's right to fight what it described as terrorism in Syria.
It accused the opposition of fabricating the accusations to divert attention from the huge losses its forces had suffered recently.
United Nations chemical weapons inspectors arrived in Syria on Sunday with a mandate to investigate three locations where chemical weapons were allegedly used, including the northern town of Khan al-Assal, where some 26 people were killed in March.
Earlier, White House spokesman Josh Earnest said in a statement: "The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons, near Damascus earlier today.
"We are formally requesting that the United Nations urgently investigate this new allegation. The UN investigative team, which is currently in Syria, is prepared to do so, and that is consistent with its purpose and mandate."
The alleged attack comes a year after US President Barack Obama warned the Syrian government that using chemical weapons would cross a "red line".
UK Foreign Secretary William Hague said that if confirmed the attacks would mark a "shocking escalation in the use of chemical weapons in Syria".
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