Syria's main opposition group accused the government of "massacring" more than 1,300 people in chemical weapons attacks near Damascus, as the UN Security Council called for "clarity" and expressed "strong concern" over the allegations.
The accusation was strongly denied by Damascus. It came as a team of UN inspectors was in Syria to probe previous allegations of chemical weapons strikes levelled against both sides during the 29-month conflict.
Following an emergency meeting of the UN Security Council in New York, the council's president, Argentina's envoy Maria Cristina Perceval, said: "There must be clarity on what happened and the situation must be followed carefully."
She added that members "welcomed the determination of the secretary general to ensure a thorough, impartial and prompt investigation".
The 15-nation body expressed "strong concern" about the allegations and agreed that any chemical weapons use is "a violation of international law".
Western governments demanded immediate access for the inspectors to investigate the new allegations. Russia, a longstanding Damascus ally, echoed the call for an inquiry but said it suspected "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, which could not be independently confirmed, was vehemently denied by the Syrian regime, which said it was intended to hinder the UN weapons inspectors' work.
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 from the "brutal use of toxic gas by the criminal regime".
In one video posted on YouTube, children are seen receiving first aid in a field hospital, notably oxygen to help them breathe. Doctors appear to be trying to resuscitate unconscious children.
Another video showed what activists said was a case of hysteria following a chemical strike in the eastern suburbs.
A young girl held her head in her hands and frantically repeated "I'm alive", as a man in a white coat tried to comfort her.
-- 'Not totally convinced' --
But 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."
John Hart, head of the Chemical and Biological Security Project at Stockholm International Peace Research Institute, said none of the alleged victims showed "pinpoint pupils ... this would indicate exposure to organophosphorus nerve agents."
The opposition National Coalition's George Sabra said more than 1,300 people had been killed in what he described as a "coup de grace that kills all hopes for a political solution in Syria".
But state news agency SANA dismissed the reports as "totally false" and "an attempt to prevent the UN commission of inquiry from carrying out its mission."
UN chief Ban Ki-moon's office said he was "shocked" by the reports and that talks were already underway with President Bashar al-Assad's government on securing access to the alleged attack sites.
Ban's deputy, Jan Eliasson, said it represented a "serious escalation" and called for a ceasefire.
The head of the UN inspection mission, Ake Sellstroem, was in talks with Damascus "on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident," a UN statement said.
White House spokesman Josh Earnest demanded the inspectors be given "immediate access to witnesses and affected individuals" and "the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government."
Washington has previously described chemical weapons use as a red line that might prompt it to intervene militarily in Syria.
But responding to the latest reports, State Department spokeswoman Jen Psaki stressed: "I'm not talking about red lines. I'm not having a debate or conversation about red lines ... I'm not setting red lines."
British Foreign Secretary William Hague said: "I hope this will wake up some who have supported the Assad regime, to realise its murderous and barbaric nature."
German Chancellor Angela Merkel was quoted as saying it would be a "frightful crime" if verified.
And Laurent Fabius, the French foreign minister, called the alleged attack a "terrible tragedy ... that is probably without precedent since what we saw with Saddam Hussein in Iraq."
Moscow, which has said it has proof of chemical weapons use by the rebels in March, reacted with scepticism.
The timing of the allegations as UN inspectors began their work "makes us think that we are once again dealing with a premeditated provocation," said the foreign ministry.