Syrian opposition groups called for international action on Friday after United States said its intelligence indicates President Bashar Assad's regime has used chemical weapons.
The American government likened the accusation to false U.S. claims of weapons of mass destruction used to justify the 2003 invasion of Iraq.
Washington's declaration was its strongest so far, a report by the Associated Press said, although the administration said it was still working to pin down definitive proof - holding back from saying Damascus had outright crossed what President Barack Obama has said would be a “red line” prompting tougher action.
The rebels accused regime forces of firing chemical agents on at least four occasions since December, killing 31 people in the worst of the attacks, and warned that world inaction would only encourage Assad to use them on a larger scale.
The Obama administration said Thursday that intelligence indicates government forces used the nerve gas sarin in two attacks.
The regime countered that it was the rebels who fired chemical weapons - pointing to their capture of a chemical factory last year as proof of their ability to do so. On Friday, government officials repeated denials the military had used the weapons.
Both sides have used the issue to try to sway world opinion.
“The red line has been crossed, and this has now been documented by the international community. We hope the U.S. will abide by the red line set by Mr. Obama himself,” Loay al-Mikdad, a spokesman for the Free Syrian Army, the umbrella group for rebel fighters, told The Associated Press.
“We need urgent action, otherwise Bashar Assad will not hesitate to use his entire chemical and unconventional weapons stockpile against the Syrian people,” he said.
Most Assad opponents say the U.S. and its allies should now arm the rebels in response to regime use of chemical weapons, a step Washington has been reluctant to take for fear the weapons will end up in the hands of Islamic hard-liners.
Some have urged international airstrikes against regime warplanes and rocket launchers that have wreaked havoc on rebel forces. Few, however, advocate direct international intervention on the ground.
At the White House, Obama said Friday that any use of chemical weapons by Syria would be a “game changer,” though he cautioned the United States needs more evidence that Assad has used the deadly agents against his people.
Obama promised a “vigorous investigation” into the reports, adding that he awaits a “definitive judgment” on whether the Syrian regime used chemical weapons against rebel fighters before taking action, AFP reported the White House as saying.
“We’re working to establish credible and corroborated facts,” White House spokesman Jay Carney told reporters. “The president wants the facts,” he added, saying there was no timeline for further action by the United States.
Carney also said that options for dealing with Syria’s use of chemical weapons “include” but are “not exclusive” to military force.