BEIRUT (Reuters) - 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 that operates in rebel-held areas, said government helicopters had dropped barrel bombs containing chlorine on the Sukari neighborhood in eastern Aleppo.

The Syrian government has denied previous accusations it used chemical weapons during the five-year-old civil war. The Syrian army could not be immediately reached for comment on the latest allegations.

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 70 cases of suffocation.

A United Nations and Organization for the Prohibition of Chemical Weapons inquiry seen by Reuters 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.

"Unimaginable crimes are occurring in Aleppo ... pro-government aerial bombardments cause mass civilian casualties," Commission Chairman Paulo Pinheiro told reporters in Geneva. "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.

Government forces put eastern 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 Syrian conflict has killed more than 250,000 people and forced more than 11 million from their homes.

(Reporting by John Davison; additional reporting by Stephanie Nebehay in Geneva.; Editing by Larry King)