Entire families had gathered in the streets before dawn hoping to leave the ravaged city after an agreement announced the night before to evacuate civilians and rebels.
The first departures had been expected around 5:00 am local time but there was no movement, and a few hours later, fierce fighting shook the city.
The landmark evacuation deal, brokered by Russia and Turkey, came after the army seized more than 90 per cent of east Aleppo from the rebels.
But it appeared increasingly fragile by Wednesday afternoon as the government and the rebels, as well as their foreign allies, traded accusations.
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Turkish President Recep Tayyip Erdogan said he would speak with Russian counterpart Vladimir Putin later in bid to rescue the deal.
"The situation on the ground is very fragile and complicated," he said.
After hours of quiet, government air strikes and heavy tank fire resumed in the last pocket of rebel-held territory in east Aleppo, an AFP correspondent reported.
The Syrian Observatory for Human Rights reported "very intense clashes on every front line" and said two people had been killed in rebel-held areas.
State television said rebel rocket fire on government-controlled areas had also resumed, killing at least seven people.
Moscow, a staunch ally of President Bashar al-Assad, said Damascus resumed its assault on Aleppo after rebels violated the ceasefire.
Foreign Minister Sergei Lavrov said Moscow expected rebel resistance in the city to end in the next "two to three days".
Rebels and a source close to the regime told AFP that the evacuation had been suspended after objections from the Syrian government.
But Yasser al-Youssef, a political official from the Nureddin al-Zinki rebel group, said the regime and its ally Iran were trying to add "new conditions" to the agreement.
"They want to link this deal to other issues, including the areas of Fuaa and Kafraya," he added, referring to two government-held Shiite-majority villages in northwestern Syria that are under rebel siege.
Turkey too accused Assad's regime and its supporters of blocking the deal.
Before the fighting resumed, AFP's correspondent saw crowds of civilians gathered in the streets of rebel areas from the early hours, some clutching bags of belongings, to await evacuation.
Many were hungry, after weeks without regular meals because of dwindling food supplies caused by the army's siege.
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