A suspected chlorine gas attack on an opposition-held neighborhood 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' organization which operates in rebel-held areas, said government helicopters had dropped barrel bombs containing chlorine on the Sukari neighborhood in Aleppo's eastern sector.

The Syrian government has denied previous accusations of using chemical weapons during the five-year-old civil war. The Syrian army could not be immediately reached for comment.

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 the violence using sources on the ground, said medical sources had reported 70 cases of suffocation.

A United Nations and Organization for the Prohibition of Chemical Weapons (OPCW) inquiry seen by Reuters last month found that Syrian government forces were responsible for two toxic gas attacks in 2014 and 2015 involving the use of chlorine.

The Civil Defence accused the government of another chlorine attack in August.

Aleppo has been one of the areas hardest hit by escalating violence in recent months after a partial truce brokered by the United States and Russia in February crumbled.

Government forces put east Aleppo under siege on Sunday for a second time since July after advancing against rebels on the city's outskirts. The city has long been divided between government and opposition areas of control.

The conflict has killed more than 250,000 people and forced more than 11 million from their homes.