The UN is investigating evidence of a toxic gas attack on a rebel-held area of the Syrian city of Aleppo, BBC News reports.

Rebels said the attack - which reportedly left four people dead and many injured - was carried out by government forces using chlorine gas.

The UN special envoy for Syria said a chlorine attack, if confirmed, would amount to a "war crime".

Footage obtained by the BBC shows people with breathing difficulties receiving treatment at a hospital.

Men, women and children are shown being fitted with oxygen masks by medical staff.

The gas is thought to have been chlorine dropped in a barrel bomb, said the Syrian Civil Defence - volunteer emergency response workers who operate in opposition-held areas.

UN special envoy Staffan de Mistura confirmed the global body's experts were investigating reports of a gas, believed to be chlorine, being dropped on Aleppo.

"There is a lot of evidence that it actually did take place," he told reporters. "If it did take place, it is a war crime and as such it would require everyone... to address it immediately."