BEIRUT — Syrian activists and rescue workers in the rebel-held part of the contested city of Aleppo said that government warplanes dropped suspected chlorine bombs Tuesday on a crowded neighborhood, injuring dozens.

The report could not be independently verified and it was not clear how it was determined that chlorine gas was released.

Accusations involving use of chlorine and other poisonous gases are not uncommon in Syria’s civil war, and both sides have denied using them while blaming the other for using it as a weapon of war. Last month, there were at least two reports of suspected chlorine attacks in Aleppo also, while the Syrian government also blamed the opposition for using the gas.

In Tuesday’s attack, a medical report from one of the hospitals in the besieged eastern rebel-held part of Aleppo was shared with journalists via text messages. It said at least 71 persons, including 37 children and 10 women, were treated for breathing difficulties, dry cough, and that their clothes smelled of chlorine. The report said 10 of the patients are in critical care, including a pregnant woman.

Ibrahem Alhaj, a member of the Syria Civil Defense first responders’ team, said he got to the scene in the crowded al-Sukkari neighborhood shortly after a helicopter dropped barrels containing what he said were four chlorine cylinders. He said he himself had difficulty breathing and used a mask soaked in salt water to prevent irritation.

At least 80 civilians were taken to hospitals and treated for breathing difficulties, he said. A video by the rescuers shows children crying and men coughing.

“Most of those injured were women and children,” he told The Associated Press in a telephone interview. “It is a crowded neighborhood.”

The head of the Britain-based Syrian Observatory for Human Rights said 70 people suffered from breathing difficulties after a barrel bomb attack in al-Sukkari on Tuesday. The Observatory’s chief, Rami Abdurrahman, said he could not ascertain if it was chlorine gas attack.

Chlorine gas is a crude weapon that can be fatal in high concentrations. In lower doses, it can damage lungs or cause severe breathing difficulties and other symptoms, including vomiting and nausea.

A team of international inspectors determined in late August that the Syrian government and Islamic State militants were responsible for chemical attacks carried out in 2014 and 2015.

Fighting in Aleppo has not let up despite international efforts to establish a cease-fire.