Syrian Civil Defence and Syrian Observatory for Human Rights say regime helicopters dropped chlorine bombs causing at least 80 people to suffer breathing difficulties. The Syrian regime has denied the claims.
A suspected chlorine gas attack on an opposition-held neighbourhood in the Syrian city of Aleppo caused dozens of cases of suffocation on Tuesday, rescue workers and a monitoring group said.
The Syrian Civil Defence, a rescue workers' organisation that operates in opposition-held areas, said regime helicopters had dropped barrel bombs containing chlorine on the Sukari neighbourhood in eastern Aleppo.
The Syrian regime, which has repeatedly denied previous accusations it used chemical weapons during the five-year-old civil war, has brushed off the claims.
The Civil Defence said on its Facebook page that 80 people had suffocated. It reported no deaths. It posted a video showing wheezing children doused in water using oxygen masks to breathe.
The British-based Syrian Observatory for Human Rights, which tracks Syrian violence using sources on the ground, said medical sources had reported 80 cases of suffocation.
A United Nations and Organization for the Prohibition of Chemical Weapons inquiry last month found that Syrian regime forces were responsible for two toxic gas attacks in 2014 and 2015 involving chlorine.
The Civil Defence accused the regime of two other suspected chlorine gas attacks in August. The United Nations Commission of Inquiry on Syria said it was investigating an August incident.
"In government-held areas, indiscriminate ground shelling (by) armed groups ... is also killing scores of civilians," he added.
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.
Regime forces put eastern Aleppo under siege on Sunday for a second time since July after advancing against opposition on the city's outskirts. The city has long been divided between regime and opposition areas of control.
The Syrian conflict has killed more than 250,000 people and forced more than 11 million from their homes.