Four killed in suspected gas attack on Aleppo

Those killed were believed to have inhaled chlorine gas and died from poisoning, the manager of al-Quds hospital Hamza Khatib told Reuters news agency.

At least 55 people were injured by the gas and Khatib had persevered evidence of the attack, including patients clothing, he told Reuters.

Three deaths and 22 injuries were reported by the civil defence rescuers after barrels of gas, believed to be chlorine, were dropped on the Zubdiya neighbourhood in rebel-held Aleppo, Reuters reported.

A woman and a child died of suffocation after helicopters dropped explosive barrels on the neighbourhoods of Seif al-Dawla and Zubdiya, the Syrian Observatory for Human Rights said.

Unconfirmed reports over the use of chlorine gas on rebel-held areas in eastern Aleppo emerged after the offensive on the city began in late June.

Rebels and activists have reported barrel bombs containing poisonous gas were dropped on residential areas around Aleppo, but the lack of chemical labs or independent testers made it difficult to verify these claims.

A world's chemical weapons watchdog raised concerns earlier this month over the reports of chlorine gas attacks around Aleppo.

"These reports are of great concern," Ahmet Uzumcu, the head of the Organisation for the Prohibition of Chemical Weapons [OPCW], said in a statement.

The UN-backed group based in The Hague "continues to examine any credible reports" of chemical weapons use, the statement said.

Uzumcu added that under international conventions the use of chemical weapons "by anyone under any circumstances" is seen "as reprehensible and wholly contrary to the legal norms established by the international community."

Syrian regime and rebels have traded accusations over which side is using toxic gas in their shelling.