The gas is believed to have been dropped alongside barrel bombs on the Zubdiya neighbourhood, which is in the rebel-held part of the city.

Hamza Khatib, manager of Al Quds hospital, told Reuters that he had recorded four deaths and 55 injuries.

Mr Khatib said he was saving pieces of patients' clothing and pieces of the bombs as evidence.

Syria Civil Defence, a rescue service operating in rebel-held parts of Syria, said they had recorded three deaths and 22 injured.

The British-based Syrian Observatory for Human Rights said a woman and her child were among the dead.

A video was posted online by the Aleppo Media Centre, an opposition news portal, which showed a child and adults wearing breathing apparatus.

Two men reported a strong smell of gas and said they saw barrel bombs before people began to suffer breathing and eye problems.

Both sides deny using chemical weapons but United Nations investigators say that sarin gas was used in Eastern Ghouta three years ago.

This killed an estimated 1,429 people, including at least 426 children.In late 2015, mustard gas was used in the conflict, confirmed by the Organisation for the Prohibition of Chemical Weapons.

It is not clear in either case which side was to blame.

Aleppo is in the north of Syria and is the subject of a bitter fight between the rebel forces and those under the command of Syrian President Bashar al Assad.

Last week, rebels broke through a month-long government siege in the city's east, where an estimated 250,000 are living.

A safe corridor for civilians and aid has not yet been created.

(SKY)