An alleged chemical weapons attack near Aleppo yesterday, for which the Syrian regime and the opposition traded accusations of responsibility, almost certainly did not feature a lethal agent proscribed under international convention, say chemical weapons experts after considering the available evidence.

Video footage and eyewitness accounts suggest that if a chemical agent was used in a missile attack on Khan al-Aasal that reportedly killed 31 people and wounded more than 100, it was most likely a riot-control agent designed to cause irritation, which is not generally lethal.

“In the end, all I can say with confidence is that whatever the conventional or non-conventional munition was, it was not a CW [Chemical Weapons] agent as defined by the CWC [Chemical Weapons Convention],” says Charles Blair, senior fellow for state and non-state threats at the Washington-based Federation of American Scientists.

Eyewitnesses reported a powerful explosion yesterday morning in Khan al-Aasal, a village southeast of Aleppo where regime and rebel forces have battled for control.

The regime blamed Syrian opposition rebels for firing a chemically-tipped missile, while the Free Syrian Army accused Damascus of launching a Scud missile fitted with a chemical warhead. The Syrian military has fired several Scud ballistic missiles at rebel strongholds in northern Syria since December.

Securing Syria’s chemical weapons arsenal has been a matter of international concern since the uprising evolved into an armed conflict in late 2011. There are at least four suspected sites where chemical weapons are manufactured and as many as 50 storage facilities. Diplomatic and rebel sources have claimed that some of the chemical weapons stockpile has been moved to new facilities, namely in the coastal region where support for the Assad regime still runs high.

The US has warned Syria that the use of chemical weapons would constitute a “red line” requiring unspecified action. The international community is also concerned that chemical weapons could fall into the hands of radical groups.

In another indication of second thoughts over the validity of the alleged chemical weapons claim, Russia appeared to backtrack today from its initial endorsement of the Syrian regime’s position that rebels were responsible for the use of chemical weapons in Khan al-Aasal.

“The story concerning the use of chemical weapons must be meticulously investigated,” Gennady Gatilov, the Russian deputy foreign minister, said on Twitter. “For now, there is no unequivocal evidence about this.”

The United States has repeated its warning of “consequences” should chemical weapons be employed in a conflict that already has killed more than 70,000 people in the past two years. But US Ambassador to Syria Robert Ford, who was recalled to the US in 2011, told a House of Representatives hearing today that there was no evidence that chemical weapons had been used in Khan al-Aasal.

“So far, we have no evidence which substantiates the reports that chemical weapons were used yesterday. But I want to underline that we are looking very carefully at these reports,” he said.

Syrian television showed crowded hospital scenes with dozens of people being treated for apparent respiratory problems. But the footage of the victims and the lack of pictures from the scene of the blast itself undermined the mutually-traded accusations of a chemical weapons attack.

“I am not convinced that the footage and pictures I have seen prove a CW attack,” wrote Jean Pascal Zanders, a senior research fellow at the Paris-based European Union Institute for Security Studies, on the Arms Control Law blog. “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 attack. Definitely not mustard; definitely not a nerve agent.”

Syria is suspected of possessing one of the largest stockpiles of chemical weapons in the world, which allegedly includes highly toxic VX and Sarin nerve agents and tissue-blistering mustard gas. There have been persistent – but unproven – allegations of chemical weapons use in Syria. Although the potent nerve and blistering agents are the greatest cause of concern, there have been no credible reports of their use so far. Most allegations suggest the exploitation of a milder agent.

In January, Foreign Policy magazine cited a US diplomatic report that the Syrian army appeared to have used Agent 15, an incapacitating nerve agent, in Homs in December.

The Monitor has heard similar unproven allegations from Syrian refugees and rebel fighters of a paralyzing agent being used near Qusayr, a rebel-held town five miles north of the border with Lebanon. During a nighttime battle in mid-January in Jusiyah, a village south of Qusayr on the border Awith Lebanon, rebel fighters were allegedly incapacitated by a smoke from a bomb they believe was dropped by a passing jet. Fighters who came to the assistance of their comrades said they found men lying “paralyzed” on the ground, some choking and most unable to speak.

“They were alive but it was like they were lifeless. Some had burns on their faces and there was a strange smell,” says Ahmad, a 20-year-old rebel combatant from Qusayr who was part of the rescue effort.

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However, requests for further evidence have so far gone unfulfilled. The Khan al-Aasal attack, which eyewitnesses reported left a chlorine smell in the air and unexplained powder on the ground, remains similarly inconclusive.

"Until someone goes to the site [in Khan al-Aasal] and collects what is alleged to have been the powder responsible, this is likely another case where somthing odd occurred but its nature was never determined," says Mr. Blair of the Federation of American Scientists.