Syrian rescue workers say dozens of civilians suffocated Tuesday after government military helicopters dropped barrel bombs containing chlorine gas on an opposition-held area of the besieged city of Aleppo. The neutral Syrian Civil Defense organization said 80 people had suffocated as a result of the attack—but reported no deaths. The group released a video showing wheezing children with oxygen masks struggling to breathe. The British-based Syrian Observatory for Human Rights confirmed the attack but put the number of victims at 70. Although the Syrian government has repeatedly denied using chemical weapons in the five-year war, a United Nations and Organization for the Prohibition of Chemical Weapons investigation released in August found Bashar al-Assad’s government forces were responsible for at least two gas attacks in 2014 and 2015 involving chlorine.