At least four people died, and many suffered breathing difficulties when gas, believed to be chlorine, was dropped alongside barrel bombs on a neighbourhood of the Syrian city of Aleppo on Wednesday, a hospital and a civil defence group said.
Hamza Khatib, the manager of Al-Quds hospital in Aleppo, told a Reuters photographer the hospital had recorded four deaths from gas poisoning and 55 injuries. Seven people were still receiving hospital treatment.
Khatib said he was preserving pieces of patients clothing and fragments from the barrel bombs as evidence for analysis.
Syria Civil Defence, a Syrian rescue service operating in rebel-held territory, said it had recorded three deaths and 22 injuries after a barrel containing a gas suspected of being chlorine fell on the Zubdiya neighbourhood of rebel-held Aleppo.
The group, which describes itself as a neutral band of search and rescue volunteers, said it could not independently verify the nature of the gas.
The Syrian Observatory for Human Rights, a war monitor, said helicopters dropped explosive barrels on the neighbourhoods of Seif al Dawla and Zubdiya, leading to the death of a woman and her child from suffocation.
The northern city of Aleppo, Syria's most populous before the war, is split into rebel- and regime-held districts. Capturing the whole city would be a major prize for Syria's Bashar al-Assad in the country's five-year-old conflict.
A fierce battle for control of the city broke out on Friday when rebels staged a major assault to break through a month-long regime siege of the city's rebel-held east, where about 300,000 people are thought to be living.
Fighters managed to pierce the ring of regime-controlled territory, but a safe corridor for civilians and aid has not yet been established.
There have been unconfirmed reports among activists and residents of chlorine gas falling on rebel-held east Aleppo since the rebel offensive.
The Aleppo Media Centre, an online opposition news portal for the city, posted a video that it said was of victims of the gas attack: a child and adults wearing breathing apparatus. Two men interviewed said barrel bombs were dropped, and there was a strong smell of gas. People then began to suffer breathing and eye problems.
Western powers say the Syrian regime has been responsible for chlorine and other chemical attacks. The government has accused rebels of using poison gas.
UN investigators established that sarin gas was used in Eastern Ghouta in 2013. The United States accused Damascus of that attack, which it estimates killed 1,429 people, including at least 426 children. Damascus denied responsibility and blamed rebels.
Later that year the United Nations and the Syrian regime agreed to destroy the state's declared stockpile of chemical weapons, a process completed in January 2016.
The Organisation for the Prohibition of Chemical Weapons confirmed in late 2015 that sulphur mustard, commonly known as mustard gas, had been used for the first time in the conflict, without saying which party in the many-sided conflict it thought had used it.