BEIRUT — A Syrian rescue worker says three civilians, a mother and two children, died in a suspected chlorine gas attack on an opposition-held district in the city of Aleppo.

Khaled Harah, a first responder, says a government helicopter dropped four barrel bombs on Wednesday night on the neighborhood of Zabadieh and that one of them released chlorine gas.

The report, which was posted online on Thursday, 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 neighborhood. It had reports of two killed and several people suffering breathing difficulties.

The Observatory made no mention of chlorine gas.