Latest: Mother and children among dead in suspected chlorine gas attack in Aleppo

Update11.18am: Three civilians - a mother and two children - have died in a suspected chlorine gas attack on an opposition-held district in the city of Aleppo, a Syrian rescue worker said.

Khaled Harah, a first responder, said a government helicopter dropped four barrel bombs on the Zabadieh neighbourhood last night, and one of them released chlorine gas.

The report, which was posted online today, could not be independently verified and it was not clear how it was determined that chlorine gas was released.

The Britain-based Syrian Observatory for Human Rights, an activist group that tracks the civil war in Syria, also reported that government barrel bombs struck the neighbourhood.

It had reports of two people killed and several people suffering breathing difficulties.

The Observatory made no mention of chlorine gas.

Earlier:

At least four people are dead after a suspected chlorine gas attack on the besieged Syrian city of Aleppo.

The gas is believed to have been dropped alongside barrel bombs on a rebel-held part of the city.

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

One local doctor told Sky News he was saving pieces of patients' clothing as evidence of the gas attack.