The use of chemical weapons in Syria would constitute a "crime against humanity", UN chief Ban Ki-moon said on Friday, adding there was "no time to lose" in probing alleged attacks which the opposition says killed hundreds.
Ban described reports of the incidents near Damascus on Wednesday as "very alarming and shocking" and urged the regime to allow a United Nations inspection team, already on the ground in Syria, to begin an investigation without delay.
Footage distributed by activists, showing unconscious children, people foaming around the mouth and doctors apparently administering oxygen to help them breathe, has triggered revulsion around the world.
Ban's comments, at a United Nations event in Seoul, piled more pressure on Syrian President Bashar al-Assad after French President Francois Hollande denounced the "likely" use of chemical weapons.
"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," Ban said. "Such a crime against humanity should result in serious consequences for the perpetrator.
"There is no time to waste," Ban said, adding that he had instructed his envoy for disarmament affairs, Angela Kane, to travel to Damascus immediately.
"I can think of no good reason why any party -- either government or opposition forces -- would decline this opportunity to get to the truth of the matter."
The United States said it has yet to "conclusively determine" chemical weapons were used. President Barack Obama has ordered US spy agencies to urgently probe the claims, aides said.
Damascus denied it unleashed chemical weapons, particularly at a time when the UN 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, a senior security source said.
The opposition National Coalition says more than 1,300 people were killed in gas attacks southwest and east of the capital.
Syrian activist Abu Ahmad, speaking to AFP over the Internet from Moadamiyet al-Sham, the rebel-held town southwest of Damascus where the deadliest attack allegedly took place, said he helped bury dozens of civilians whose bodies were "pale blue", and who died of "suffocation".
Videos posted online by activists have provoked shock and condemnation around the globe.
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.
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 US State Department said Obama had instructed intelligence services to gather information about the claims.
"Right now, we are unable to conclusively determine CW (chemical weapons) use," said State Department spokesperson Jen Psaki.
But she said the administration had been focused since reports of the attack broke on efforts to "nail down the facts."
One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".
US Secretary of State John Kerry spoke with his British counterpart William Hague late on Thursday about the developments, the Foreign Office said.
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.
One million Syrian children now live as refugees abroad, UN children's agency UNICEF said on Friday. Two million more have been displaced within their homeland's borders.
"This one millionth child refugee is not just another number. This is a real child ripped from home, maybe even from a family, facing horrors we can only begin to comprehend," agency chief Anthony Lake said.
The UN refugee agency has registered all one million children by name.