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 includingsarin, sometimes not," said Britain's ambassador to the United Nations, Mark Lyall Grant.
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.