BEIRUT/GENEVA/ISTANBUL: 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.

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

In Tuesday’s attack, a medical report from one of the hospitals in the 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.

Ibrahim 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 AP over the phone. “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 Tuesday. The Observatory’s chief, Rami Abdel-Rahman, said he could not ascertain if it was chlorine gas attack.

A team of international inspectors determined in late August that the Syrian government and Daesh (ISIS) militants were responsible for chemical attacks carried out in 2014 and 2015. But the U.N. Security Council failed to agree on whether to impose sanctions on the government in line with a September 2013 resolution authorizing sanctions.

The resolution followed Syria’s approval of a Russian proposal to relinquish its chemical weapons stockpile and join the Chemical Weapons Convention. That averted a U.S. military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta. Russia, a close Syrian government ally, has blocked sanctions against President Bashar Assad’s government. Fighting in the deeply contested city of Aleppo has not let up despite international efforts to establish a cease-fire. Syrian pro-government forces backed by airstrikes launched a wide offensive in the city Sunday, capturing areas they lost last month and besieging rebel-held neighborhoods once more after a breach in the siege a month earlier.

U.N. investigators Tuesday said aerial bombardment by Syrian forces and their ally Russia were mostly to blame for swelling numbers of civilian casualties in Syria’s devastating conflict.

The U.N. Commission of Inquiry on Syria criticized all parties in the bloody war over a clear increase in “indiscriminate attacks on civilians,” citing attacks on medical workers and facilities, blocked humanitarian convoys, enforced disappearances and summary executions.

Investigator Vitit Muntarbhorn told reporters that aerial bombardments by “pro-government forces ... cause the most civilian casualties and damage to the civilian infrastructure, particularly in Idlib and Aleppo.”

When asked to clarify who exactly the “pro-government forces” referred to, commission chief Paulo Sergio Pinheiro said “the forces that are in the air are Russian and Syrian forces.”

A Turkish spokesman said Tuesday that Turkey was pushing for a cease-fire in Aleppo that would extend through the Muslim religious holiday of Eid al-Adha, due to begin Monday. Spokesman Ibrahim Kalin said President Recep Tayyip Erdogan spoke to his U.S. and Russian counterparts during the G-20 meeting in China about the cease-fire.

The U.S. State Department said Washington is making progress in talks with Russia over how to achieve a cessation of hostilities in the Syria conflict, but is not willing to settle for less than a clear path toward achieving that aim.

State Department spokesman Mark Toner initially said the United States would not settle for a less than ideal agreement, but he later clarified that Washington needed a clear path forward toward achieving a nationwide cessation of hostilities to reach a deal on Syria.

Erdogan also repeated calls for a safe zone to be established between the Syrian towns of Azaz and Jarablus in Aleppo province, to protect civilians.

Turkey sent tanks into Syria last month to support rebel forces against Daesh in the town of Jarablus. It expanded its operation into nearby Al-Rai over the weekend. Two Turkish soldiers were killed and five were wounded in a missile attack by Daesh Tuesday – the first Turkish casualties caused by the militants in Turkey’s 2-week-old incursion into Syria.