"There are no images of the site of the attack; just of some affected people. These people do not show outward symptoms of a CW (chemical weapon) attack. Definitely not mustard; definitely not a nerve agent," wrote Jean Pascal Zanders, senior research fellow at the European Union Institute for Security Studies.

"There are far too many people, including non-medical staff, around the affected persons. Apart from a surgical mask, nobody wears any protective garment or gas masks. If there would have been a CW attack with one of the agents known (or believed) to be in Syria's arsenal, then most of the people present would have been fatally or seriously contaminated."

He added that, during the Arab uprisings, witness reports cited chemical attacks, but none had been confirmed. "People are exposed to a wide range of toxicants in today's battlefield," he said in an e-mail. "Furthermore, once a rumor gets around, people are more likely to think that they suffer from symptoms similar to the ones being rumored."

Not the first round of claims

U.S. officials pointed to previous claims that the Syrian government used chemical weapons, which were found to be false after extensive investigation.

The Syrian government did not use chemical weapons against residents of Homs in a December attack, a U.S. State Department investigation showed, but did apparently misuse a riot-control gas in the incident, according to senior U.S. officials.

The officials said the State Department launched a probe from its consulate in Istanbul, Turkey, after reports from doctors and activists that dozens of people suffered nervous system, respiratory and gastrointestinal ailments after inhaling the gas in Homs on December 23.

The civil war -- which began two years ago after a government crackdown on Syrian protesters -- has left around 70,000 people dead and uprooted more than 1 million others, the United Nations has said.