Syrian opposition groups called yesterday for international action after the Obama administration said US intelligence indicates President Bashar Assad's regime has used chemical weapons.
However the government likened the accusation to false US claims of weapons of mass destruction used to justify the 2003 invasion of Iraq.
Washington's declaration was its strongest so far, although it 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 Mr Assad to use them on a larger scale.
The Obama administration said on 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.
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 US 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, said.
"We need urgent action, otherwise Bashar Assad will not hesitate to use his entire chemical and unconventional weapons stockpile against the Syrian people."
Most Assad opponents say the US 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 advocate direct international intervention on the ground.
At the White House, Mr Obama said any use of chemical weapons by Syria would be a "game changer," though he warned the US needs more evidence that Mr Assad has used the deadly agents against his people.
He said the US, along with the United Nations, would seek to "gather evidence on the ground" in Syria to solidify intelligence assessments.
Use of chemical weapons would bring a frightening wild-card element to Syria's two-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, neighbourhoods and cities with artillery, rockets and warplanes.
A Syrian government official denied that it carried out any chemical attacks, saying Mr Assad's military "did not and will not use chemical weapons even if it had them".
The army, he said, can reach any area in Syria it wants without them.