(AP) France said Tuesday it has confirmed that the nerve gas sarin was  used "multiple times and in a localized way" in Syria, including at  least once by the regime. It was the most specific claim by any Western  power about chemical weapons attacks in the 27-month-old conflict.
Britain later said that tests it conducted on samples taken from Syria also were positive for sarin.
The  back-to-back announcements left many questions unanswered, highlighting  the difficulties of confirming from a distance whether combatants in  Syria have crossed the "red line" set by President Barack Obama. The  regime of Syrian President Bashar Assad has refused to allow U.N.  investigators into the country.
The French and British findings,  based on samples taken from Syria, came hours after a U.N. team said it  had "reasonable grounds" to suspect small-scale use of toxic chemicals  in at least four attacks in March and April.
The U.N. probe was  conducted from outside Syria's borders, based on interviews with doctors  and witnesses of purported attacks and a review of amateur videos from  Syria. The team said solid evidence will remain elusive until inspectors  can collect samples from victims directly or from the sites of alleged  attacks.
Some experts cautioned that the type of evidence  currently available to investigators — videos, witness reports and  physiological samples of uncertain origin — leaves wide doubts.
At  the same time, forensic evidence of alleged chemical weapons use is  fading away with time, and the longer U.N. inspectors are kept out of  Syria, the harder it will be to collect conclusive proof, they said.
Syria  is suspected of having one of the world's largest chemical weapons  arsenals, including mustard and nerve gas, such as sarin. In recent  weeks, the regime and those trying to topple Assad have increasingly  used accusations of chemical weapons use as a propaganda tool, but have  offered no solid proof.
In the West, meanwhile, the lack of  certainty about such allegations is linked to a high stakes political  debate over whether the U.S. should get more involved in the Syria  conflict, including by arming those fighting Assad.
Obama has  been reluctant to send weapons to the Syrian rebels, in part because of  the strong presence of Islamic militants among them. Obama has warned  that the use of chemical weapons or their transfer to a terrorist group  would cross a "red line," hinting at forceful intervention in such an  event.
Yet he has insisted on a high level of proof, including a  "chain of custody," that can only come from on-site investigations  currently being blocked by the regime.
In Tuesday's announcement  about sarin, French Foreign Minister Laurent Fabius said his government  had analyzed several samples, including some brought back from Syria by  reporters from the Le Monde newspaper.
He said that there was "no  doubt" that at least in one case, the regime and its allies were  responsible for the attack. "We have integrally traced the chain, from  the attack, to the moment people were killed, to when the samples were  taken and analyzed," Fabius told the TV station France 2.
He said  a line was crossed and that "all options are on the table," including  intervening "militarily where the gas is produced or stored."
In  London, Britain's Foreign Office said samples from Syria were tested at a  government laboratory and that the presence of sarin was confirmed. It  did not say when or where the samples were obtained.