The United Nations hopes to carry out the first medical evacuations from Aleppo on Friday, if a ‘humanitarian pause’ in the Syrian army's Russian-backed assault on the city holds.

Despite a drop in violence after the unilateral ceasefire took effect on Thursday, there was little sign civilians were heeding calls to leave opposition-held areas of the city, and Russia accused the rebels of intimidation.

The unilateral ceasefire was initially described as lasting just 11 hours, but Russian Defence Minister Sergei Shoigu announced late Thursday that it would be extended ‘by 24 hours,’ leaving it unclear exactly when it will end.

East Aleppo, which the rebels captured in 2012, has been under siege by the army since mid-July and has faced devastating bombardment by the government and its ally Russia since the launch of an offensive to retake the whole city on September 22.

Nearly 500 people have been killed, more than a quarter of them children, since the assault began. More than 2,000 civilians have been wounded.

The scale of the casualties has prompted outrage in the West, with Washington saying the bombardment amounted to a possible war crime.

Russia announced a halt to its air strikes from Tuesday and the unilateral ceasefire from Thursday.

- Few leaving -

The Syrian army has opened eight corridors across the front line for the more than 250,000 civilians in rebel-held areas to leave but so far almost none have taken up the offer.

‘There has been no movement in the corridors in the eastern district. For the moment, we haven't seen any movement of residents or fighters,’ said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

An AFP correspondent on the government-held side of one crossing in the Bustan al-Qasr district also reported no movement at all early on Friday.

He reported just eight people passing through it on Thursday.

There has been no air or artillery bombardment of east Aleppo since the ceasefire began but sporadic clashes have continued on the front line, some of them near the evacuation corridors.

The Observatory reported overnight skirmishes on the front line in the Old City and in al-Zahraa district.

UN humanitarian task force chief Jan Egeland said that Russia, the Syrian government, and rebels had given permission for medical evacuations to start on Friday, adding that the UN also hopes to deliver food to the besieged east.

No UN aid convoy has entered Aleppo since July 7 and food rations will run out by the end of October, UN chief Ban Ki-moon warned on Thursday.

The United Nations has been criticised by the Syrian opposition for focusing more on enabling people to leave than providing relief supplies to allow them to stay.

A joint statement from the Syrian National Coalition and the Free Syrian Army said UN policy ‘is flawed, and instead of preventing forced displacement, it plays into the Assad regime's plans to empty Aleppo.’

It accused the world body of becoming a ‘tool in the hands of Russia.’

- Moscow blames rebels -

Moscow hit back, accusing rebels of preventing residents from leaving.

‘The rebels are violating the ceasefire and preventing the evacuation of the population,’ Foreign Minister Sergei Lavrov told his US counterpart John Kerry by telephone on Thursday.

Moscow and Damascus have called on civilians to leave so that the offensive can focus on fighters of former Al-Qaeda affiliate, the Fateh al-Sham Front.

The two governments have said repeatedly that there will be no let-up in their offensive until the jihadists leave.

But at a meeting of the UN General Assembly on Thursday, US ambassador Samantha Power accused Russia and Syria of targeting civilians.

‘Let us be real: terrorists are not the ones dropping bunker-busting bombs on hospitals and civilian homes in eastern Aleppo, nor are they besieging civilians there,’ she said.

‘The Assad regime and Russia are.’

Moscow has shown no sign that its intervention in support of its Syrian ally will end any time soon.

A Russian aircraft carrier battle group is currently in the North Sea en route from the Baltic to the eastern Mediterranean.

But Western governments have been divided over how to respond.

After meeting in Brussels on Thursday, European Union leaders backed down from an explicit threat of sanctions against Russia but warned they would consider ‘all available options’ if atrocities continue.