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

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

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 much it would require everyone... to address it immediately."

Fighting has escalated in Aleppo in recent days, with rebels severing the government's main route to the west of the city.

The offensive sought to break a siege by pro-government forces, who encircled the east in July with the support of Russian aircraft.

Source: MENA