Multiple reports of fighting in Aleppo signal the collapse of a ceasefire brokered by Turkey and Russia meant to allow for the withdrawal of the remaining rebels and tens of thousands of civilians.
The evacuation was delayed over demands by pro-government forces. The shelling resumed on the morning of 14 December and green buses meant for the evacuation, which were waiting at the rendezvous point on the edge of the rebel enclave, were driven back to their depots, according to the Associated Press.
Pro-government forces struck the remaining rebel-held district with dozens of rockets since mid-morning, activists reported.
"The bombardment is scarcely to be believed on the eastern neighborhoods and until now 40 people have been wounded," said Ibrahim al-Haj, a spokesman for the Syrian Civil Defence.
"They are using all forms of weapons," he added.
A citizen journalist reporting for a Turkish agency told AP a rocket crashed beside him at around 10.15 am (8.15 GMT), as he shared an audio recording of the explosion.
Syrian journalist Zourir AlShimale, from eastern Aleppo, broadcast a video on Twitter describing the heavy bombing. "The attack is still ongoing", he said, as explosions can be heard in the background.
Turkish President Tayyip Recep Erdogan said he will discuss the matter of the ceasefire with Russian President Vladimir Putin on 14 December and accused President Bashar al-Assad of crimes against humanity.
"This truce is perhaps the last hope for innocent Aleppo. The human corridor should be opened immediately and without obstruction, and innocent people should be allowed to leave eastern Aleppo. The Assad administration has committed crimes against humanity," he said in a speech.
Turkish Foreign Minister Mevlut Cavusoglu accused the Syrian government and its allies of trying to ruin the ceasefire. "We see now that the regime and other groups are trying to obstruct this (deal)," he said, quoted by the state-run Anadolu Agency. "This includes Russia, Iran, forces supported by Iran and the regime."
The Russian Defense Ministry blamed the rebels for resuming the fighting. The ministry said in a statement the rebels were trying to break through Syrian government positions to the north-west.
Late on 13 December, the UN envoy for Syria, Staffan de Mistura, called for immediate humanitarian access to the former rebel enclave. De Mistura was at the Security Council where an emergency meeting for Aleppo was held, in which the US ambassador to the UN Samantha Power fiercely condemned Assad and its allies for what the UN called "a complete meltdown of humanity".
She said: "Three member states of the UN [are] contributing to a noose around civilians. It should shame you. Instead, by all appearances, it is emboldening you. You are plotting your next assault. Are you truly incapable of shame?"
UN agencies reported that pro-government forces had been summarily killing civilians in Aleppo, including women and children.
Unicef added that as many as 100 children may be trapped inside a building under heavy attack in the city.