US Secretary of State John Kerry said Wednesday he had asked his French counterpart Laurent Fabius to share evidence of Syria's apparent use of chemical weapons against rebel forces.
"I asked him... whether he could send us the information that shows us the chain of custody of that evidence, so we know precisely where it came from," Kerry said on the sidelines of an Organization of American States meeting.
President Barack Obama's administration is facing new pressure to act on a perceived violation of its "red line" against the Syrian regime's use or movement of chemical weapons, following the new information released in Paris.
Fabius said Tuesday, citing tests carried out by a French laboratory, that the deadly nerve agent sarin was used "several times in a localized manner" but did not give details of where, when or by whom it was used.
White House spokesman Jay Carney did not give a definitive response Tuesday to the French claim, but said Washington was working with France, Britain and the Syrian opposition to probe the possible use of chemical weapons.
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On Wednesday, Kerry said Washington was completing its own analysis and that he was "very comfortable" with the US government's "own calendar" on how and when to act.
"Make no mistake whatsoever: the president's red line is real. He has a set of options which are alive and waiting," Kerry said.
Ake Sellstrom, head of the UN investigation on chemical weapons that has been blocked by Syria, said Wednesday that only an on-site investigation could prove whether the arms had been used.
Sellstrom confirmed, in a statement released by UN spokesman Martin Nesirky, that France had provided new information about the use of chemical weapons when he visited Paris on Tuesday.
"Mr Sellstrom cautions that the validity of the information is not ensured in the absence of convincing evidence of the chain-of-custody of the data collected. In this regard, he reiterates his belief that on-site activities are essential if the United Nations is to be able to establish the facts," said the statement.
The Syrian government asked the UN to set up the investigation but has since refused to let the experts enter the country, insisting that they only investigate government claims that opposition rebels had used chemical weapons.
Sellstrom and his experts have been examining evidence provided by other countries and interviewing witnesses who have fled Syria, according to diplomats.