Shelling has resumed in the remaining rebel part of eastern the city of Aleppo, despite a ceasefire deal to allow for the evacuations of the opposition fighters and tens of thousands of Syrian civilians, according to activists.

Aleppo media activist Mahmoud Raslan said he was reporting for a Turkish agency when a rocket crashed beside him at around 10.15am local time on Wednesday. He shared an audio recording of the explosion with the Associated Press.

The Britain-based Syrian Observatory for Human Rights said an explosion was heard in the rebel-held Saif al-Dawleh district at around the same time.

Pro-government forces have trapped thousands of civilians and gunmen in eastern Aleppo under a crushing bombardment over the past weeks as they pushed to clear the northern city of the opposition.

The implementation of the deal struck on Tuesday to evacuate civilians and rebels from the enclave, signalling a surrender by the opposition, was delayed on Wednesday morning.

The withdrawal had been expected to start early in the morning, hours after the remaining rebel factions reached the ceasefire deal An opposition official told the Associated Press on Wednesday that there were 800 sick and wounded people requiring immediate medical evacuation from eastern Aleppo.

The last-minute deal was mediated by Ankara and Moscow as the rebel enclave rapidly dissolved and ceded more and more territory in the face of the brutal advance by Syrian forces, backed by Russia and assisted by Shiite militias from Lebanon, Iraq and Afghanistan.

Late on Tuesday, the UN envoy for Syria, Staffan de Mistura, called for immediate access to the former rebel enclave to confirm the end of military operations and to oversee the safe departure of tens of thousands of civilians and opposition fighters from the last sliver of eastern Aleppo into which they had been squeezed by the advancing government forces.

Mr De Mistura was at the Security Council where an emergency meeting for Aleppo was held.

The pan-Arab al-Mayadeen TV broadcast on Wednesday footage of Syrian government buses idling at an agreed-on evacuation point. The cause of the delay was not immediately clear but the TV report said it was expected to last at least another couple of hours.

It said the buses are prepared to move 5,000 fighters and their families to Atareb, an opposition-held town in the north-western Aleppo countryside.

Brita Haj Hassan, a Syrian opposition official living in exile, told the Associated Press from Luxembourg that the UN and other intermediaries had informed the opposition that the evacuation had been delayed until Thursday.

However, there was no comment from the government, the United Nations or aid groups on the ground. Haj Hassan, who is on the opposition's local council for Aleppo, said he blamed Russia and Iran for the delay.

The dramatic developments surrounding Aleppo - which would restore the remainder of what was once Syria's largest city to President Bashar Assad's forces after months of heavy fighting and a crippling siege - followed reports of mass killings by government forces closing in on the final few blocks still held by the rebels.

UN Secretary-General Ban Ki-moon told the emergency meeting late on Tuesday that he had received "credible reports" of civilians killed by pro-government forces as they swept into the last rebel areas in Aleppo.

"To the Assad regime, Russia and Iran - three member states behind the conquest of and carnage in Aleppo - you bear responsibility for these atrocities," said US Ambassador Samantha Power.

Bashar al-Ja'afari, Syria's UN ambassador, denied any mass killings or revenge attacks, but added that it was Syria's "constitutional right" to go after "terrorists" - a reference to all opposition fighters.

"Aleppo has been liberated from terrorists and those who toyed with terrorism," he said. "Aleppo has returned to the nation."