At least three people were killed when a gas, believed to be chlorine, was dropped on a neighborhood in the Syrian city of Aleppo on Wednesday, according to a hospital and a local rescue group.

The Syrian Civil Defence, a volunteer rescue group operating in rebel-held territory, told Al Jazeera it had recorded three deaths and at least 25 injuries after a barrel containing a gas suspected of being chlorine fell on the Zubdiya neighbourhood of rebel-held Aleppo.

"At about 7:30pm on Wednesday, an explosive barrel was dropped from a helicopter on the Zubdiya neighbourhood. One woman suffocated to death due to gas inhalation, along with her 10-year-old daughter and four-year-old son," Khaled Khaled, an Aleppo-based member of the rescue group, told Al Jazeera.

The group, also known as the White Helmets, said it could not independently verify the nature of the gas.

Hamza Khatib, the manager of Al Quds hospital in Aleppo, told a Reuters news agency photographer that the hospital had recorded four deaths from gas poisoning and 55 injuries.

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Khatib said he was preserving clothing and fragments from the barrel bombs as evidence for analysis.

Helicopters also dropped explosive barrels on the neighborhoods of Seif al Dawla and Zubdiya late on Wednesday evening, according to the the Britain-based Syrian Observatory for Human Rights.

The northern city of Aleppo, Syria's most populous before the war, is split into rebel and government-held districts.

A fierce battle for control of the city has been raging for nearly a week, after rebels staged a major assault last week to break a month-long government siege of the rebel-held east, home to nearly 300,000 people.

Rebel fighters managed to break the siege over the weekend, but a safe corridor for civilians and aid has not yet been established.

A three-hour ceasefire, announced by the Russian army, is set to begin on Thursday at 10am local time to allow humanitarian convoys to enter the city, although the United Nations has said it was not nearly long enough to help trapped civilians.

Chemical attacks

There have been unconfirmed reports from activists and residents of chlorine gas falling on rebel-held east Aleppo since the rebel offensive began.

Last week, there were reports of a chlorine attack in the Aleppo neighbourhood of al-Qatarji, although no one was seriously injured.

On the day that alleged attack was reported, at least 33 people, including 18 women and 10 children, were taken to hospital after a chlorine attack in Saraqeb, a town in Idlib province.

Government and opposition forces have both denied using chemical weapons during the war.

Western powers say the government has been responsible for chlorine and other chemical attacks, and the government and Russia have accused rebels of using poison gas.

UN investigators established that sarin gas was used in Eastern Ghouta in 2013. The United States accused the Syrian government 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 UN and the Syrian government agreed to destroy the state's declared stockpile of chemical weapons, a process completed in January 2016.