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Syria's main opposition group accused the regime of using chemical weapons on Wednesday to strike rebel areas near Damascus in a "massacre" that left more than 1,300 people dead.
Videos distributed by activists, the authenticity of which could not immediately be verified, showed medics attending to suffocating children and hospitals being overwhelmed.
More footage showed dozens of people laid out on the ground, among them many children, some of them covered in white sheets.
The claim of a chemical attack, which could not be independently verified, was vehemently denied by the Syrian regime which said it was intended to hinder the mission of UN chemical weapons inspectors now in the country.
The Local Coordination Committees (LCC), a network of activists, reported hundreds of casualties in the "brutal use of toxic gas by the criminal regime".
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.
The opposition National Coalition's George Sabra, who spoke to reporters in Istanbul, labelled the attack as a "coup de grace that kills all hopes for a political solution in Syria".
State news agency Sana said "reports on the use of chemical weapons in Ghouta are totally false. It's an attempt to prevent the UN commission of inquiry from carrying out its mission."
British Foreign Secretary William Hague said his country will refer the charges of a chemical weapons strike to the Security Council.
The Arab League urged the inspectors to visit the site immediately "to see the reality of the situation and investigate the circumstances of this crime.
