The regime in war-torn Syria came under intense pressure Thursday to allow UN inspectors to probe an alleged chemical weapons attack near Damascus which the opposition says killed hundreds.
Footage distributed by activists showing unconscious children, people foaming around the mouth and doctors apparently giving them oxygen to help them breathe has triggered revulsion around the world.
The United Nations formally asked Syria to authorise UN experts to probe the allegations, and Secretary General Ban Ki-moon decided to send a special envoy to Damascus.
French President Francois Hollande denounced the "likely" use of chemical weapons while the United States said it has yet to "conclusively determine" such weapons were used.
Damascus denied it unleashed chemical weapons, particularly at a time when a UN team was in Syria to inspect three sites where other such attacks allegedly took place.
It would be "political suicide" to go ahead with such an attack, said a senior security source.
The opposition National Coalition says more than 1,300 people were killed by poisonous gases in a rebel-held town southwest of the capital.
Syrian activist Abu Ahmad, speaking to AFP over the Internet from Moadamiyet al-Sham which reportedly bore the brunt of the alleged chemical attacks, said he helped bury dozens of civilians whose bodies were "pale blue", and who died of "suffocation."
A UN spokesman said: "The secretary general now calls for the mission, presently in Damascus, to be granted permission and access to swiftly investigate the incident which occurred on the morning of 21 August 2013.
"A formal request is being sent by the United Nations to the government of Syria in this regard. He expects to receive a positive response without delay."
Ban would also send to Damascus at an undisclosed time Angela Kane, the UN High Representative for Disarmament Affairs.
And the UN rights chief Navi Pillay condemned the reported deaths and urged the Syrian government and opposition forces alike to allow the UN inspectors to visit the sites without "delay or obfuscation."
Pillay also called for an end to fighting, referring to reports of new air raids by regime forces on rebel positions, including those in areas where the chemical attacks allegedly took place.
The Syrian Observatory for Human Rights, which has a network of activists and medics on the ground, said air strikes and bombardments were launched across Damascus province while fighting rage in the southwest of the capital.
Videos posted online by activists of the alleged chemical attacks have provoked shock and condemnation around the globe.
One footage shows people foaming at the mouth and bodies laid out in long lines, while in another a doctor is apparently trying to resuscitate some children as others are given oxygen to breathe.
Another video shows what activists said was a case of hysteria following a chemical strike in the eastern suburbs.
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None of the videos could be verified but AFP analysed one of the most striking pictures showing the bodies of children using specialised software.
The analysis showed the picture was not manipulated and was taken, as presented, on August 21.
Former US Army Chemical Corps officer Dan Kaszeta said "it would be relatively hard to fake" the amount of video footage that has surfaced.
"There's a lot of stuff that goes on in that video and a lot of the victims sadly are children, and it's hard to get small children to consistently fake things," said Kaszeta, an independent consultant.
If confirmed, the attack would be the largest scale use of chemical weapons since Saddam Hussein's Iraqi forces attacked the Kurdish town of Halabja in 1988.
The activist Abu Ahmad said the army fired "rockets... mounted with chemical warheads to target Moadamiyet al-Sham."
"We buried the victims in the middle of the night, because the area we use as a cemetery is within sight of the army," Abu Ahmad said.
Another activist, Abu Adel, said "people who were on the streets felt nauseous and started collapsing", after one alleged attack, and that he also felt ill.
"My eyes became sore, and it became difficult to breathe. I had a very, very bad headache."
The Britain-based Observatory said 170 people died Wednesday: 71 men, 34 women, 25 children and 40 rebels.
Experts said convulsions, pinpoint pupils and laboured breathing seen in footage of alleged victims could be symptoms of nerve gas.
But they also insisted only blood and urine samples gathered from the victims could provide definitive proof.
"I was sceptical about the claims of nerve agent neurotoxicants (but) I have revised my position on that a bit on the basis of footage I have seen... where a number of symptoms consistent with organophosphorus poisoning" were apparent, said chemical weapons specialist Jean Pascal Zanders.
Organophosphorus is a chemical compound used in nerve agents like sarin and kills by asphyxiation.
The State Department said President Barack Obama had instructed intelligence services to gather information about the claims.
One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences."
More than 100,000 people have been killed in Syria's 29-month war, the UN says. Millions more have been forced to flee their homes.