Syria's main opposition group has accused the government of "massacring'' more than 1300 people in chemical weapons attacks near Damascus, saying many of the victims choked to death.
The accusation came as a team of UN inspectors was in Syria on Wednesday to probe previous allegations of chemical weapons strikes levelled against both sides during the 29-month conflict.
Western governments demanded immediate access for the inspectors to investigate the new allegations.
Russia, a longstanding ally of the Damascus regime, echoed the call for an inquiry but said it suspected a "provocation'' by the opposition and its foreign backers.
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 chemical weapons use was vehemently denied by the Syrian regime, which said it was intended to hinder the work of the UN weapons inspectors already in the country.
Opposition sources accused the army of multiple chemical weapons strikes - one in Moadamiyet al-Sham, southwest of Damascus, and more in the capital's eastern suburbs.
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 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''.
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 one video, children are seen being given first aid in a field hospital, notably oxygen to help them breathe, while doctors appear to be trying to resuscitate unconscious children.
Several specialists in the impact of chemical weapons said the video evidence was not entirely convincing.
"At the moment, I am not totally convinced because the people that are helping them are without any protective clothing and without any respirators,'' said Paula Vanninen, director of Verifin, the Finnish Institute for Verification of the Chemical Weapons Convention.
"In a real case, they would also be contaminated and would also be having symptoms.''
The opposition National Coalition's George Sabra said more than 1300 people had been killed in what he described 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 (the Damascus suburbs) are totally false. It's an attempt to prevent the UN commission of inquiry from carrying out its mission.''
The U.N. Security Council held emergency consultations on the latest alleged use of chemical weapons in Syria, and Secretary-General Ban Ki-moon said he is determined to ensure a ``thorough investigation'' of all reported incidents.
U.N. deputy spokesman Eduardo del Buey said the secretary-general is "shocked'' at the alleged use of chemical weapons on the eastern suburbs of Damascus.
U.N. Deputy Secretary-General Jan Eliasson was briefing the council on the incident.
Council members said they didn't know if there would be a statement afterward from the U.N.'s most powerful body.
The head of the UN inspection mission, Ake Sellstrom, was "in discussions with the Syrian government on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident'', a statement said.
Washington demanded that the inspectors be given unfettered access.
"For the UN's efforts to be credible, they must have immediate access to witnesses and affected individuals, and have the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government,'' said White House spokesman Josh Earnest.
Washington has previously described chemical weapons use as a red line that might prompt it to intervene militarily in Syria.