A Chlorine gas attack on an opposition-held neighbourhood in the Syrian city of Aleppo caused dozens of cases of suffocation.
The Syrian government helicopters had dropped barrel bombs containing chlorine on the Sukari neighbourhood in eastern Aleppo.
The Syrian government has denied previous accusations it used chemical weapons during the five-year-old civil war.
The British-based Syrian Observatory for Human Rights, which tracks Syrian violence using sources on the ground, said medical sources had reported 70 cases of suffocation.
A United Nations and Organisation for the Prohibition of Chemical Weapons inquiry last month found that Syrian government forces were responsible for two toxic gas attacks in 2014 and 2015 involving chlorine.
The Civil Defence accused the government of two other suspected chlorine gas attacks in August.
The United Nations Commission of Inquiry on Syria said it was investigating an August incident.
Aleppo has been one of the areas hardest hit by escalating violence in recent months after the collapse of a partial truce brokered by the United States and Russia in February.
The Syrian conflict has killed more than 250,000 people and forced more than 11 million from their homes.
