The US state department says it has "indications" that a toxic chemical likely to be chlorine was used in areas of rebel-held Syria earlier this month.
A toxic industrial chemical may have been used in the town of Kfar Zeita in Syria's Hama province - an area of the country controlled by forces opposed to President Bashar al-Assad - State Department spokesperson Jen Psaki said on Monday.
She added that the US is working alongside the United Nations and the Organization for the Prohibition of Chemical Weapons to get further information.
French authorities reached the same conclusion, according to the country's president Francois Hollande who told the Europe 1 radio station that there were "several elements" suggesting the use of chemical weapon usage.
The attacks reportedly took place on April 11 and 12. Amateur pictures and video quickly appeared supposedly showing yellow-tinged smoke rising from an explosion and the apparent aftermath of the attack, including civilians coughing and exhibiting suffocation symptoms.
Both opposition activists and state media agreed on casualty figures of more than 100 wounded as well as that the agent used was chlorine. Each blamed the other for the attacks.
Opposition activists alleged that helicopters dropped barrel bombs containing chlorine. "Regime planes bombed Kfar Zeita with explosive barrels that produced thick smoke and odors and led to cases of suffocation and poisoning," said Rami Abdel Rahman, director of the opposition Syrian Observatory for Human Rights at the time.
However, Syrian state TV blamed the attacks on Jabhat al-Nusra, al Qaeda's one-time Syrian affiliate. "There is information that the terrorist al-Nusra Front released toxic chlorine..." it said in a report, according to AFP.
'Our assessment is it is concentrated chlorine dropped from helicopters... That could only be the regime.'
It has been impossible to verify either claim. However, some argue that because the attacks seem to have been made from either a plane or a helicopter, it is highly unlikely that they were made by al-Nusra, which is not known for its air force.
"It's been interesting to see the remains of chlorine cylinders, and maybe even an ammonia cylinder, in improvised barrel bombs reportedly dropped from helicopters in towns and villages in Idlib and Hama," Eliot Higgins, who blogs under the alias Brown Moses and focuses on weaponry used in the Syrian conflict, told VICE News.
"All reports from across these locations claim they've been dropped from helicopters, and only government factions have access to a helicopter fleet."
US officials have apparently reached the same conclusion. "Our assessment is it is, at a minimum, concentrated chlorine dropped from helicopters... That could only be the regime," said one official, according to CNN.
Numerous videos have appeared online since the April 11 attacks, apparently showing parts of chlorine cylinders in improvised explosive devices in rebel-held areas and more suffocation victims. While the contents of the cylinders cannot be verified, the injuries depicted in the videos all appear to be consistent with chemical exposure, Higgins said.
"There's been a lot of these videos from different sites over the last two weeks, and it seems unlikely this is just the locals deciding to post a bunch of fake chemical exposure videos online."
Last August, a previous attack, reported to have killed as many as 1,400, took place in Damascus. The government again claimed rebel forces were responsible, but many - including much of the international community - blamed the regime. In the aftermath, the US threatened military action unless the Syrian government surrendered its chemical weapon stockpile.
Damascus subsequently did so, although it missed the February 5 deadline to move its 1,300 tons of chemical substances out of the country and has since promised to do so by the end of April.
Opposition activists also accused Assad's forces of fresh gas attacks in Damascus on April 11 and 16. They also posted video footage of men receiving medical treatment after one supposed attack in the city's Harasta neighborhood.
Higgins stresses that these too cannot be verified but adds that if they did take place as claimed, they would fit a different profile than the incidents in Kfar Zeita. "The attacks in Harasta seem to fit a pattern of small scale chemical attacks on front-line positions seen elsewhere over the past months, and I think that's something separate from these chlorine attacks."