Syrian military forces dropped chlorine gas in rebel-held residential areas of Aleppo at least eight times late last year, killing four children and five other civilians in a protracted battle to retake the city, Human Rights Watch said in a report released Monday.
Roughly 200 people in eastern Aleppo were injured in the attacks, in which helicopters dropped chlorine cylinders in neighborhoods where government forces planned to advance, the report said.
"The pattern of the chlorine attacks shows that they were coordinated with the overall military strategy for retaking Aleppo, not the work of a few rogue elements," Ole Solvang, deputy emergencies director at Human Rights Watch, said in the report.
It said the chlorine attacks started on Nov. 18 and continued until Dec. 9, days before the fight for Aleppo ended with a cease-fire and wide-scale evacuations of fighters and civilians.
If verified, the attacks would be a violation of the 1993 Chemical Weapons Convention, an international treaty Syria signed in 2013 that bans the use of chemical weapons.
Documenting the attacks was difficult because government forces confiscated phones and laptops and hospital workers had to leave records behind during the evacuation, the report said. Its conclusions are based on video footage, photographs and interviews with nearly two dozen witnesses, including medical workers, first-responders and residents.
Witnesses said that victims suffered symptoms consistent with chlorine attacks, including trouble breathing, nausea, fainting and foaming at the mouth. Video clips published with the report show plumes of green gas billowing from the city and in a neighborhood, as well as women and children being treated with oxygen masks.
Human Rights Watch called for the United Nations Security Council to impose sanctions on senior Syrian government leaders.
"Allowing the Syrian government to flaunt this prohibition with impunity runs the risk of implicitly condoning Syrian chemical attacks and undermining one of the most agreed-upon weapon bans in the world, potentially lowering the threshold for other countries to do the same," Solvang said in the report.
The government has routinely denied using chemical weapons in the civil war.
Last year, a U.N. team investigating chemical weapons concluded that Syrian government forces used chlorine gas three times in 2014 and 2015.
In 2013, Human Rights Watch accused Syrian government forces of using sarin gas in attacks on two Damascus suburbs, killing hundreds of civilians.
Syrian helicopters used in past chlorine attacks operated from the Hmeymim air base, which is controlled by Russia, a staunch ally of the Syrian government that has aided its war effort.
Russia — which has also signed the treaty — should have prevented chemical weapons from being used in their joint military offensive, the latest report said.