The United Nations is investigating a suspected toxic gas attack on the besieged city of Aleppo, saying it would amount to a war crime if confirmed.

At least four people have died and dozens of others - many children and babies - were injured after bombs allegedly containing chlorine were dropped on the rebel-held Zubdiya area in the city.

Children began "screaming and suffocating" amid a strong smell of gas, witnesses said.

A doctor working in Aleppo told ITV News that an estimated 60 patients suffering with "symptoms of choking and coughing" were rushed to his hospital.

Staffan de Mistura, the UN's special envoy for Syria, said there "is a lot of evidence" that a chlorine attack did take place, adding that it would be classed as a war crime if verified.

Grim details of the suspected gas attack emerged as the UN called on Syrian President Bashar al-Assad and his Russian allies to respect a 48-hour ceasefire.

"The stakes could not be higher in the coming days - millions of Syrian civilians are now in a seeming freefall," Jan Egeland, the UN 's humanitarian adviser said.

Fighting has escalated in Aleppo in recent weeks, with the Russian-backed Assad regime engaged in an intense stand-off with rebels.

ITV News has documented the battle with many observers expressing fears of a humanitarian crisis with thousands trapped inside the city with scarce resources.

Russian forces announced they would suspend fighting for three hours a day to allow humanitarian aid in to the city.

But UN officials say the offer was "not enough" to help the thousands in need of help.

Earlier, the last doctors in the rebel-held east of the city wrote to US President Obama to urge him to intervene to protect Syrian civilians and to stop the bombardment of hospitals.

Mr Obama was told to "act now to stop the bombs that continue to fall on the city and ensure they are never held under siege again".