Syria is under intense pressure to allow UN weapons inspectors into the site of an alleged chemical weapons attack near Damascus which the opposition says left hundreds dead and provoked revulsion around the world.
Damascus has flatly denied the allegations but a US official cited by the Wall Street Journal said on Thursday there were "strong indications" regime forces had indeed unleashed chemical weapons during attacks the previous day on rebel zones near the capital.
France said it is seeking a reaction with "force" if allegations by the National Coalition, the main opposition group, of a massacre involving chemical weapons are confirmed.
The Coalition said more than 1300 people had died while videos and photographs showed scenes of people foaming at the mouth and of bodies stacked up in morgues.
In one video posted on YouTube, children are seen receiving first aid in a field hospital, notably oxygen to help them breathe. Doctors appear to be trying to resuscitate unconscious children.
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Another video showed what activists said was a case of hysteria following a chemical strike in the eastern suburbs. The authenticity of the videos has not immediately been verified.
The Syrian Observatory for Human Rights reported that Syrian forces on Thursday bombed and shelled a number of rebel zones where the Coalition alleged the attacks took place the previous day.
Western governments demanded immediate access for a team of UN inspectors already in Syria to probe previous allegations of chemical weapons strikes to the sites of the alleged attacks.
The head of the UN inspection mission, Aake Sellstroem, was in talks with Damascus "on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident," a UN statement said.
White House spokesman Josh Earnest demanded the inspectors be given "immediate access to witnesses and affected individuals" and "the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government."
Washington has previously described chemical weapons use as a red line that might prompt it to intervene militarily in Syria.
The Wall Street Journal reported a senior administration official as saying the claims of a chemical weapons attack appeared to have some credibility.
"There are strong indications there was a chemical weapons attack - clearly by the government," the unnamed official said.
"But we do need to do our due diligence and get all the facts and determine what steps need to be taken."
French Foreign Minister Laurent Fabius urged a reaction "with force" if the massacre was confirmed.
"If it is proven, France's position is that there must be a reaction, a reaction that could take the form of a reaction with force," Fabius told BFM-TV.
Human rights groups backed calls for access for the UN inspectors to the sites of the alleged attacks.
Should the allegations be true, "the attacks would amount to war crimes," said Amnesty International's deputy director for the Middle East and North Africa Hassiba Hadj Sahraoui, who urged the UN inspectors to visit the site.
The UN inspectors arrived in Damascus on Sunday with a strict mandate to investigate three sites for the alleged use of chemical weapons.
A high-ranking Syrian security source described the idea that the army would use chemical weapons while UN weapons inspectors were working inside the country as "political suicide".
