The Syrian government has had little response to a new corridor opened for rebels and civilians who want to leave besieged eastern neighbourhoods of Aleppo.
The move is part of a Russia-announced pause in the fighting in the ravaged city, but the UN humanitarian agency said medical evacuations have been impossible so far because of a lack of security.
A UN official said opposition fighters are blocking medical evacuations because the government and Russia are impeding deliveries of medical and humanitarian supplies into the city.
Residents in eastern Aleppo have said many will not leave as there are no guarantees that the evacuees will not be arrested by government forces, and Russia said fighters from the Al-Nusra militant group are refusing to leave.
Even as the corridor opened along Aleppo's main artery to the north, Castello Road, intense clashes and shelling erupted in the Jobar neighbourhood in the capital of Damascus. The Britain-based Syrian Observatory for Human Rights said there were casualties among rebels and government forces.
The pan-Arab Al-Mayadeen TV aired live footage from Castello Road showing bulldozers that had opened the route. Buses and ambulances were parked by the road waiting to take evacuees.
The pause in fighting was announced by Moscow to allow the evacuation of civilians and fighters, as well as the wounded, and t he Russian military said later that the break in fighting had been extended for a third day.
Lieutenant General Sergei Rudskoi said the dawn-to-dusk "humanitarian pause" that began on Thursday will be extended for another day on president Vladimir Putin's instructions.
The UN said it had received verbal assurances for the extension until Monday, but the Kremlin did not confirm it, saying only that it is possible if militants do not abuse it.
Before the pause, Aleppo's besieged districts were subjected to relentless Syrian and Russian air strikes for weeks.
UN secretary-general Ban Ki-moon told an emergency meeting of the UN General Assembly that nearly 500 people have been killed and almost 2,000 injured since the Syrian government launched its offensive in eastern Aleppo on September 23.
On Thursday, government helicopters dropped leaflets over eastern Aleppo, saying that this is "the road to the nation".
"We are ready for help. Take the opportunity," said the leaflets, which carried an image of a green bus or a dead rebel fighter which had the words "this could be the end" underneath.
The collapse of the last truce was followed by some of the worst bombing of Aleppo in years.
In Geneva, the UN human rights chief said Aleppo is "a slaughterhouse" and urged the Human Rights Council to set aside "political disagreements" to focus on suffering civilians.
Zeid Ra'ad al-Hussein delivered the stark remarks in an address by video-conference to the 47-member UN-backed rights body as it opened a special session on Aleppo called by Britain.
Mr Zeid, a Jordanian prince, said rights violations and abuses in Syria "constitute crimes of historic proportions". He said the "collective failure of the international community to protect civilians and halt this bloodshed should haunt every one of us".
Save the Children reported widespread use of cluster bombs recently in rebel-held areas of Aleppo.
The international group said that despite humanitarian pauses, there are concerns about an increased number of children already injured by cluster bombs who may be too unwell to leave or untreatable in existing medical facilities.
Save the Children quoted the Violations Documentation Centre, a Syrian group that tracks human rights violations, as recording 137 cluster bomb attacks in Aleppo from September 10 to October 10.
It said the attacks are a 791% increase on the average of the previous eight months. Across Syria, they reported that 130 children have been killed by cluster bombs in the past year.
Despite this, Syria's ambassador in Geneva claimed the "human suffering in Aleppo is not an emergency" and lashed out at "propaganda" by Britain and other countries, alleging that they have armed al Qaida-backed fighters in Syria.
Hussam Edin Aala spoke during a special session of the UN-backed Human Rights Council, where Britain and allies are leading a push for a resolution that would call for a stepped-up UN investigation on rights abuses and a halt to air strikes.
According to a translation of his remarks, the ambassador said the suffering in Aleppo dates to mid-2012 when the city was "attacked by thousands of terrorists from (al Qaida-linked) Al-Nusra, Ahrar al-Sham and other terrorist groups".
British ambassador Julian Braithwaite retorted that Britain "is not supplying weapons to anyone in Syria".