Syrian President Bashar al-Assad's forces are believed to have dropped bombs containing chlorine gas on three rebel-held towns in Syria last month, according to evidence gathered by Human Rights Watch (HRW).
If true, the attacks violate the Chemical Weapons Convention treaty signed by the Syrian government in October 2013 following a chemical attack in August that killed as many as 1,400 people.
HRW investigated three attacks that occurred in April on the northern Syrian towns of Kafr Zita, al-Teman'a, and Telmans. After interviewing 10 witnesses, including local medics, and analyzing video and photographic evidence, the organization concluded that the attacks killed at least 11 people. Nearly 500 others exhibited symptoms corresponding to chlorine exposure, which include reddening and irritation of the eyes, vomiting, uncontrollable coughing, difficulty of breathing, and suffocation.
Video footage shows victims of the attack on Kafr Zita receiving medical attention.
The analysis strongly suggested that Syrian government forces dropped crude "barrel bombs" containing chlorine canisters on the towns from helicopters. This method for delivering makeshift explosives is widely used by Assad's men. Rebel groups do not have access to aircraft.
Nadim Houry, deputy Middle East and North Africa director at Human Rights Watch, urged the international community to take "firm collective action" in response.
"As long as the Security Council fails to penalize Syria for its flagrant violations of the Chemical Weapons Convention, these inherently indiscriminate and egregious attacks will continue," he said.
Chlorine, which has many industrial applications, is not in the CWC's Schedule 1 category of most deadly chemical weapons. The quick dispersal of chlorine is not particularly lethal, but HRW suspected that it was included in the bombs to give the impression that they contained more dangerous poisonous or toxic gas and cause panic among local residents.
The Organization for the Prohibition of Chemical Weapons said in late April that it would launch a fact-finding mission to investigate the use of chlorine gas in Syria, but it has yet to comment further on the matter.
Witnesses reported seeing clouds of yellow smoke consistent with the release of chlorine gas. Photographs and video circulated by local activists captured images of the clouds.
"I was at the hospital when the first barrel bomb fell 300 meters away," a doctor who treated victims of the Kafr Zita attack told HRW. "I went outside and saw dark yellowish smoke, which is unusual. I sent the paramedics. When I went out I smelled chlorine, but it wasn't a strong smell. I didn't think that it would really turn out to be chorine. The smell is common. It is used in households."
Opposition activists and state media have both acknowledged the chlorine attacks, but each blamed the other. Syrian state TV said that the rebel group Jabhat al Nusra, al Qaeda's Syrian affiliate, was responsible. HRW noted that fragments of chlorine canisters found at the scene indicated that they had been dropped from considerable heights, however, implicating the Syrian government's helicopters.
"All reports from across these locations claim they have been dropped from helicopters, and only government factions have access to a helicopter fleet," he said.
Assad's regime also tried blaming the large-scale chemical attack last August on rebel forces, but the international community was unmoved. In the aftermath of the attack, the United States threatened military action unless the Syrian government surrendered its chemical weapons stockpile.
The government complied, although it missed the February 5 deadline to move its 1,300 tons of chemicals out of the country as well as a subsequent April 27 extension, by which time it had removed 92.5 percent of its stockpile. It has promised to dispose of the remaining chemicals by June 30.
On Tuesday, Danish Foreign Minister Martin Lidegaard called on Assad's government to quickly resolve the "serious security problems" presented by the remaining 16 containers, which include precursors for the extremely toxic nerve agents VX and sarin.
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