The United Nations Security Council was to vote on Sunday on sending observers to Aleppo, as trapped civilians and rebels waited desperately for evacuations to resume from an opposition-held enclave in the flashpoint Syrian city.

A rebel representative told AFP an agreement had been reached to allow more people to leave the city which has been ravaged by some of the worst violence of the nearly six-year war that has killed more than 310,000 people.

But there was no confirmation from the Syrian government or its allies Russia and Iran, which are under mounting international pressure to end what US President Barack Obama denounced as the "horror" in Aleppo.