Due to come into effect at around 3pm (AEDT) the deal includes a reciprocal evacuation of neighbouring towns under rebel control.
An earlier ceasefire between Syrian government forces and rebels had broken down just hours after it started amid reports of heavy shelling, gunfire, and airstrikes.
The ceasefire, brokered by Russia and Turkey, was supposed to end years of fighting in Aleppo, giving Syrian President Bashar al-Assad his biggest victory in the conflict yet.
But Turkey's foreign minister, Mevlut Cavusoglu, says pro-government forces resumed their bombardment of a tiny pocket of rebel-held territory.
"The regime is violating the truce by shelling the neighbourhood. People were able to go to the streets for a few hours, but the shelling started. This is a provocation. They didn't want the deal to go through."
Russian foreign minister Sergei Lavrov says Syrian government forces were responding to rebel attacks.
"I hope in the next two to three days the situation in Eastern Aleppo will be resolved. We have created humanitarian corridors there and tens of thousands of civilians leave through them and have already left. They get humanitarian assistance on the way out of Aleppo. And there are passages created for the rebels who are offered to leave amicably and they are given safety guarantees so they would finally leave."
Another factor in the breakdown was Iran -- one of President Assad's main backers.
They imposed new conditions, demanding the simultaneous evacuation of wounded from two villages besieged by rebel fighters.
As a result, buses that were meant to evacuate people -- including around 5,000 rebels fighters and their families -- have left the outskirts of Aleppo empty.
The United Nations says the bombardment "most likely constitutes war crimes".
UN spokesman Stephane Dujarric said he was concerned for the safety of those still trapped inside.
"Any continued fighting leaves thousands of civilians in the direct line of fire. The safety and security of tens of thousands of men, women and children still trapped in eastern Aleppo remains precarious. We urgently call for a pause in fighting to allow people who wish to leave safely the besieged eastern Aleppo for destination of their choice. The parties must also ensure that those who have surrendered or been captured and are treated humanely and in line with international law."
But in an interview on Russian television, Syrian President Bashar al-Assad again accused western countries of hypocrisy.
"If we liberate Aleppo from the terrorists the Western officials and the mainstream media are going to be worried about the civilians. They are not worried when the opposite happens, when the terrorists are killing those civilians or attack Palmyra and start destroying the human heritage, and not only the Syrian heritage."
Syria's envoy to the United Nations, Bashar Jaafari has also condemned what he's called "fake news" surrounding the battle for Aleppo.
During an emergency meeting of the UN Security Council, Mr Jaafari held up a picture of what he claimed was a soldier helping an elderly woman leave the city's last rebel enclaves.
But it was later pointed out that the photo actually showed a member of Iraq's Shia militia helping a civilian in the city of Fallujah earlier this year.
In any case, neither the rebels nor Assad's forces appear willing to give up the fight.
And Foreign Minister Julie Bishop points out a truce in Aleppo doesn't mean an end to the Syrian war.
"Even a ceasefire now won't end this conflict because there are still parts of Syria held by the regime, parts held by armed opposition groups and parts held by ISIL (Islamic State) - the terrorist organisation. The United Nations is seized of the matter, but until such time as a permanent deal - a permanent ceasefire - can be brokered by the United States and Russia, I fear, this conflict will continue, and it's been going since 2011."
