The Syrian government is strongly suspected of dropping chlorine gas bombs on opposition-held northern areas of the country, Human Rights Watch (HRW) says, calling it a “plain violation” of international law. The government and opposition have each accused each other of dropping the bombs.

“While chlorine is a common industrial chemical, its use as a weapon is banned by the 1993 Chemical Weapons Convention, which prohibits the use of toxic properties of chemicals to kill or injure,” the group says.

“Since Syria became the 190th state party to the Chemical Weapons Convention on October 14, 2013, the convention applies to all actors in Syria under any circumstances. In addition, all states party to the Chemical Weapons Convention, including the government of Syria, should be acting to prevent and suppress any activity prohibited by the convention, including the use of chemicals as weapons.”

We asked Lama Fakih, HRW’s Beirut-based Syria-Lebanon researcher, to explain the report and its implications.

Syria Deeply: What evidence is there of chlorine attacks in the north?

Lama Fakih: The evidence we’ve compiled strongly suggests that the Syrian government has been dropping cylinders with chlorine gas embedded in them into the northern parts of Syria starting from early to mid-April. Chlorine is an industrial chemical being used as a weapon, and it is banned by the Convention Against Chemical Weapons, of which the Syrian government has been a signatory since 2013. Evidence we’ve seen suggests that chlorine canisters were dropped from the air by helicopters, and because they are the only ones with helicopters, we believe them responsible.

We made the assessment based on interviews with people on the ground, including five medics, and by reviewing video of the attacks. In some cases video actually showed the bombs dropping. We also had conversations with patients about their symptoms. Chlorine has killed at least 11 people in three towns and affected another 500 whose symptoms match the symptoms of chlorine gas exposure: redness and itchiness of the eyes, difficulty seeing, shortness of breath, uncontrollable coughing and even suffocation.

Video and photo evidence shows some of the cylinders have markings identifying them as chlorine gas. You see CL2 on the cylinders or the marking that says it’s a Chinese manufacturer that produces chlorine gas.

We’re looking at the Syrian government’s failure to comply with the chemical weapons convention. The Organisation for the Prohibition of Chemical Weapons (OPCW) inspections team is now in Syria and has been given access to conduct an investigation to see if the government has failed to comply.

We don’t have any evidence that opposition groups have used chlorine in this conflict. We have reviewed one video that shows what looks like a barrel bomb dropping and then a cloud of smoke emerging immediately afterwards. When chlorine gas is contained in a canister, you’ll see characteristic yellow smoke after the blast, and it’s clearly visible in the video. So this, along with the related statements by witnesses and individuals affected, all point towards government responsibility.

Syria Deeply: How does chlorine compare to sarin and other agents removed from Syria by the OPCW?

Fakih: It’s less deadly. You can see just by the figures that a lot of people have been injured, but the death toll has been 11. But chlorine, as a weapon, is indiscriminate, and it serves to really terrorize the local population and make people feel very insecure. The Syrian government has denied that it is responsible for the use of chlorine gas and has accused some opposition groups, including al-Nusra, of having a cache of it. Again though, the evidence suggests that these were air-dropped canisters and only the government would have the capacity to use them.