DAMASCUS: The regime in war-torn Syria came under intense pressure on 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 a US official told the Wall Street Journal there were “strong indications” the regime used them.

But Damascus denied it unleashed chemical weapons, particularly at a time when a UN team was in Syria to inspect other such attacks.

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.

An activist told AFP he helped bury dozens of civilians overnight and that their bodies were “pale blue”.

“They had died of suffocation,” Abu Ahmad told AFP over the Internet from Moadamiyet al-Sham, a town which reportedly bore the brunt of the alleged chemical attacks.

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.

There has been no immediate independent confirmation of the attack but images activists posted online showed people foaming at the mouth and bodies laid out in long lines.

In one YouTube video, children are seen receiving first aid in a field hospital while doctors appear to be trying to resuscitate others.

Another video showed what activists said was a case of hysteria following a chemical strike in the eastern suburbs.

The authenticity of the videos could not 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 told AFP from Moadamiyet al-Sham.

Another activist, Abu Adel, said that after the early morning attack, “people who were on the streets felt nauseous and started collapsing”, adding he also felt ill.

“My eyes became sore, and it became difficult to breathe. I had a very, very bad headache.”

The Syrian Observatory for Human Rights, which uses names and videos to document killings in the war, 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 United States, which has warned the use of chemical weapons could prompt military intervention in Syria, has demanded inspectors be given access to witnesses and collect evidence.

“The images we have seen are nothing short of horrifying,” said White House spokesman Josh Earnest who made the call for inspectors to work without interference from the regime.

The UN inspectors arrived in Damascus on Sunday with a strict mandate to investigate three sites for the alleged use of chemical weapons.