BEIRUT (AP) -- Syrian opposition groups called Friday for international action after the Obama administration said U.S. intelligence indicates President Bashar Assad's regime has used chemical weapons. The 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, 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.
He said the U.S., along with the United Nations, would seek to "gather evidence on the ground" in Syria to solidify intelligence assessments.
State Department spokesman Patrick Ventrell said Friday that the U.S. is "working to establish credible and corroborative facts to build on this intelligence assessment" and to definitively say "whether or not the president's red line has been crossed."
Asked about Syria's denials, he said that "if the regime has nothing to hide, they should let the U.N. investigators in immediately so we can get to the bottom of this."
Use of chemical weapons would bring a frightening wild-card element to Syria's 2-year-old civil war, which is estimated to have already killed more than 70,000 people. Throughout the conflict, civilian casualties have been heavy as regime forces batter rebel-held towns, neighborhoods and cities with artillery, rockets and warplanes.
Still, the chemical attacks the rebels claim the regime carried out, if confirmed, would appear to be relatively small-scale and localized.
Bilal Saab, director of the Institute for Near East and Gulf Military Analysis, North America, said if the regime is behind them, it may be trying to make detection as difficult as possible and to maintain plausible deniability.
"The government may also feel that the time for full-on use of chemical weapons has not come yet. It may also be indirectly communicating with Western powers and testing their resolve," he said.
If the rebels were using them as the regime claims, it would be a "strategic blunder," given how it would taint the movement, he said, adding that one possible scenario is that they were carried out by extremists within the rebel movement.
In December, after rebels seized control of a chlorine factory in Aleppo, the government warned the opposition might be planning a chemical attack to frame the regime. To back up its claims, the state-run SANA news agency pointed to videos posted on YouTube that purported to show regime opponents experimenting with poisons on mice and rabbits. The origin of the videos was not known.
It is not clear exactly how many people have died in alleged chemical attacks because of the scarcity of credible information. The Syrian government seals off areas it controls to journalists and outside observers, making details of the attacks extremely sketchy.