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BEIRUT -- 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, neighbourhoods 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.

Al-Mikdad said the opposition has documented four attacks based on air and soil samples and the blood of victims, in addition to eyewitness accounts. He said the results have been shared with Western countries, though he declined to name them.

The deadliest was on the village of Khan al-Assal near the northern city of Aleppo, where at least 31 people were killed in March.

The village is controlled by the government, and the regime accused rebels of firing a missile carrying chemical agents.

The opposition contends it was regime fire. Aleppo-based activist Mohammed Saeed said the army appeared to have hit government troops by mistake, inflicting casualties among them and then blaming the opposition. Neither side has offered evidence to back their claims.

In another alleged chemical attack, a government air raid on April 13 on the contested Sheikh Maqsoud neighbourhood in Aleppo killed at least four people and wounded more than a dozen others. Activists say doctors treating the wounded said many showed symptoms of inhaling a toxic gas, including severe vomiting and irritation to the nose and eyes.

Eyewitnesses speaking in a video allegedly taken a day after the attack and posted online by activists reported that an explosion left several people unconscious and others reporting aches and dizziness.

"There was a smell, so we went out and I felt dizzy and my eyes turned red," a young boy said.

Another video showed several people on stretchers at a hospital, some twitching and foaming at the mouth and nose.

The videos were consistent with AP reporting of an attack in the area on April 13, although it was not known if the symptoms resembled those triggered by a chemical weapons attack.

A defence analyst who viewed the video of the victims lying on stretchers after the attack said that, while it was impossible to verify that a nerve agent caused their symptoms, they appeared to be the result of something other than traditional weaponry.

"What you're immediately struck by is this is not your normal ordnance ... that it seems of a different type," said Michael O'Hanlon, a senior fellow at the Brookings Institution.

"Now whether that automatically guarantees that it is a specific nerve agent, I wouldn't go so far as to say based on my knowledge. But it does have the effect, it does have the appearance of being something caused by something besides traditional explosives or penetrating metal," O'Hanlon said.

Activists reported two other alleged chemical attacks, including one in December in the central city of Homs in which they said six rebels died after inhaling white smoke pouring from shells fired on the area.

Videos of the aftermath of that attack showed men in hospital beds coughing and struggling to breathe as doctors placed oxygen masks on their faces.

"The smell was like hydrochloric acid. People started choking and I wasn't able to breath," a man identified as a rebel said in a video posted online after the attack by activists.

The video appeared genuine and corresponded to AP reporting of violence in Homs in December, although it was impossible to verify if the symptoms were triggered by a chemical weapons attack.

The Britain-based Observatory for Human Rights, which relies on a network of activists on the ground, said the group has documented the two attacks in Aleppo province, but did not have proof of the other two.

A Syrian government official denied the government carried out any chemical attacks, saying 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.

The official spoke on condition of anonymity because he was not authorized to give official statements.

A Syrian lawmaker, Sharif Shehadeh, echoed that assertion, saying the Syrian army "can win the war with traditional weapons" and has no need for chemical weapons.

Syria's official policy is to neither confirm nor deny it has chemical weapons.

Shehadeh likened the allegations to the false accusations that Iraq possessed weapons of mass destruction that U.S. policymakers used to justify the 2003 invasion.

"What is being designed for Syria now is similar to what happened in Iraq," he said.

Following the Khan al-Assal attack, the government called for the United Nations to investigate alleged chemical weapons use by rebels.

Syria, however, has not allowed a team of experts into the country because it wants the investigation limited to the single Khan al-Assal incident, while U.N. Secretary-General Ban Ki-moon has urged "immediate and unfettered access" for an expanded investigation.

U.N. spokesman Martin Nesirky said Friday that U.N. disarmament chief Angela Kane wrote another letter to Syrian authorities on Thursday urging the government to grant access to the U.N. chemical weapons experts without conditions.