Russia on Friday urged Syria's government to cooperate with UN experts over claims it waged deadly chemical weapons attacks, as calls grew for international action over the alleged atrocity.

US President Barack Obama said the alleged use of chemical weapons in Syria was "clearly a big event of grave concern", after UN chief Ban Ki-moon said they would constitute a crime against humanity.

The UN children's agency UNICEF said, meanwhile, that one million Syrian children now live as refugees abroad as a result of the country's 29-month conflict.

Russia's Foreign Minister Sergei Lavrov, in a statement issued after he held a telephone conversation with his US counterpart John Kerry, called for rebel fighters to allow UN inspectors safe access to areas where chemical weapons are alleged to have been used.

The Syrian opposition said this week that the regime of President Bashar al-Assad, an ally of Moscow, used chemical weapons east and southwest of Damascus in attacks that killed hundreds of people.

The regime denies that it unleashed any chemical attacks.

Harrowing 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.

"It is now up to the opposition to ensure safe access for the mission to the site of the alleged incident," said Lavrov.

Both Lavrov and US Secretary of State Kerry agreed on the need for an "objective investigation," his ministry said in a statement.

"Common interest was expressed in an objective investigation by a UN expert mission, which is currently in the country, in connection with the reports about the possible use of chemical weapons in a Damascus suburb."

UN Secretary General Ban said there was "no time to lose" in probing the alleged attacks, and urged Damascus to allow a UN team already on the ground to begin an investigation without delay.

Obama said the latest opposition allegations of chemical weapons use were more serious than previous ones against Assad's regime.

"We are right now gathering information about this particular event," he said, while warning against the United States intervening hastily and getting "mired in very difficult situations"

One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".

Damascus has denied it unleashed chemical weapons, while playing down the likelihood of the UN team that is investigating three other sites going to the area of the latest alleged attacks.

"On the international level, there is increasing conviction that if there was a chemical weapons attack, it was perpetrated by terrorists, but it may be that this is a great charade," a security official told AFP.

Asked about the likelihood of the UN inspectors visiting the sites, the official said: "They are working on a programme that has been set in advance."

The opposition National Coalition says more than 1,300 people were killed in gas attacks southwest and east of the capital.

An activist speaking to AFP 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 who died of "suffocation".

Videos posted online by the activists have provoked shock and condemnation around the globe.

None could be verified but AFP analysed one of the most striking images showing the bodies of children using specialised software.

It 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 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 the footage could be symptoms of nerve gas. But they insisted only blood and urine samples 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 Nations says more than 100,000 people have been killed in Syria since its uprising flared in March 2011. Millions more have been forced to flee their homes.

UNICEF said on Friday that one million Syrian children now live as refugees abroad, and two million more have been internally displaced.

"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.

UN-Arab League envoy Lakhdar Brahimi warned the conflict is "the biggest threat to peace and security in the world".