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.
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."