Russia hit out Friday at calls for force to be used after Syria's opposition and European states accused the war-torn country's regime of killing hundreds in chemical weapons attacks.
US President Barack Obama said the alleged use of chemical weapons was "a big event of grave concern," as Britain accused Damascus of unleashing the weapons and France called for "force" if the claims were confirmed.
As the regime's allies and foes traded barbs, UNICEF said one million children have fled Syria in what the UN children's agency called a "tragic milestone" in the 29-month conflict.
Opponents of President Bashar al-Assad said his forces used chemical weapons east and southwest of Damascus in attacks Wednesday that killed hundreds. The regime denies the accusations.
Activists released harrowing footage showing unconscious children, people foaming around the mouth and doctors apparently giving them oxygen has triggered revulsion around the world.
British Foreign Secretary William Hague blamed Syria and demanded it grand immediate access to UN inspectors who are already in the country to probe three other sites.
"We do believe this is a chemical attack by the Assad regime on a large scale, but we would like the United Nations to be able to assess that," said Hague.
Russia also urged Damascus to cooperate with the UN experts but dismissed calls for use of force against its ally.
"Against the background of another anti-Syrian wave of propaganda, we believe calls from some European countries to apply pressure on the UN Security Council and already now take a decision on the use of force are unacceptable," its foreign ministry said.
It described the attack as "clearly provocative in nature", charged that Internet footage said to implicate the regime had been posted before it took place and accused rebels of obstructing a probe.
Foreign Minister Sergei Lavrov called on the Syrian government to cooperate with the UN chemical experts and urged rebels also to give them safe access to the sites of the alleged attacks.
Lavrov gave his statement after talks with US Secretary of State John Kerry during which they agreed on the need for an "objective investigation," according to Moscow.
UN chief Ban Ki-moon said there was "no time to lose" in probing the alleged attacks.
Obama said the latest 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, meanwhile, played down the likelihood of the UN team investigating the latest claims.
"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.
The UN inspectors on the ground "are working on a programme that has been set in advance," said the official.
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 suffocated civilians.
The shocking Internet videos and opposition claims could not 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.
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.
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 including one million children, UNICEF said.
"One million is more than the number of children living in Wales. One million children is more than the number of children living in Los Angeles and Boston combined," said UN high commissioner for refugees Antonio Guterres.
Meanwhile, twin car bombings in the Lebanese port of Tripoli, across the border from Syria, killed at least 29 people on Friday and wounded 500, the Red Cross said.