At least four people died and many suffered breathing difficulties when a gas, believed to be chlorine, was dropped alongside bombs in Aleppo.

Hospital workers said at least four people had died from gas poisoning and another 55 were injured when the chlorine and barrel bombs hit the Zubdiya neighbourhood of the Syrian city on Wednesday.

Aleppo was until Monday in the grips of attempts to lift a government-led siege which effectively cut off 250,000 people from food and medical supply.

Clothing from patients caught up in Wednesday's attack, the latest in claims that chemicals are being used against civilians, are being preserved in order to analyse for evidence.

Hamza Khatib, the manager of Al Quds hospital in Aleppo, told Reuters the hospital had recorded four deaths from gas poisoning and 55 injuries. Seven people were still receiving hospital treatment.

Syria Civil Defence, a Syrian rescue service operating in rebel-held territory, said it had recorded three deaths and 22 injuries.

The group was unable to verify the nature of the gas.

Among the dead were a woman and her child who died from suffocation, according to The Syrian Observatory for Human Rights.

A fierce battle for control of the city broke out on Friday when rebels staged a major assault in order to break the month-long government siege of east Aleppo - home to roughly 250,000 people.

While the siege appears to have been lifted, a safe corridor for civilians in and out of the city and for aid has not yet been established.

Meanwhile, there have been unconfirmed reports among activists and residents of chlorine gas falling on the east half of the city since the insurgency began.

Russia has in the meantime announced that its forces will hold fire for three hours each day from Thursday to let aid into Aleppo.