Updated 7:23 p.m. ET
PARISFrance 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. report, however, did not say conclusively which side had used the weapons.
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 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 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.
"We need to expand the evidence we have, we need to make it reviewable, we need to have it corroborated," White House press secretary Jay Carney said on Tuesday in reaction to France's claims.
He said the administration is "highly skeptical" of claims that opposition fighters have used chemical weapons, but so far Washington officials have refused to conclusively pin blame for chemical weapons use on the Assad 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 the presence of sarin was confirmed. It did not say when or where the samples were obtained.
Britain has evidence suggesting a number of different chemical agents have been used, "sometimes including sarin, sometimes not," said Britain's ambassador to the United Nations, Mark Lyall Grant.
In late May, the European Union lifted its embargo against sending arms to Syria. While France and Britain -- both vocal Assad detractors -- had been the two biggest proponents of ending the embargo, Austrian Foreign Minister Michael Spindelegger told reporters that during negotiations France and Britain had agreed not to deliver any weapons until Aug. 1.
White House spokesman Jay Carney, speaking before the British announcement, said the French report is "entirely consistent" with the Obama administration's own findings, but added more work needs to be done to establish who is responsible for the use of the toxic substances and when they were used.
"We need more information," he said.
Russia, meanwhile, has rejected intelligence the U.S. provided last month suggesting the Assad regime used chemical weapons on its own people, American officials said. A U.S. diplomatic delegation that was sent to Moscow failed to persuade Russian officials and prompted no change in the Kremlin's support for Assad, said the officials, who spoke on condition of anonymity because they weren't authorized to speak publicly on the matter.
Experts disagreed on whether the latest chemical weapons allegations mean Obama's red line has been crossed.
"The verdict is still open," said Jean Pascal Zanders, an independent chemical weapons consultant, speaking before the French and British announcements.
Zanders said that while claims of chemical weapons use cannot be ignored, the details of the alleged attacks often don't correspond to the purported symptoms shown in videos or reported by witnesses.
Analyst Michael Eisenstadt said he believes Obama's red line "has indeed been crossed on a number of times, as there are persistent reports of limited, continued use of chemical weapons from various sources that seem fairly credible."