The head of the United Nations Human Rights Council on Friday accused pro-Syrian government forces of committing war crimes in their assault on rebel-held eastern Aleppo, as UN-organised medical evacuations due to take place on Friday morning were delayed because of insecurity.

In an impassioned speech, Zeid Raad al-Hussein said the bombardment of eastern Aleppo by the Syrian and Russian air forces had turned rebel-held areas into a "gruesome locus of pain and fear".

"The ancient city of Aleppo, a place of millennial civility and beauty, is today a slaughterhouse – a gruesome locus of pain and fear, where the lifeless bodies of small children are trapped under streets of rubble and pregnant women deliberately bombed," said the Jordanian diplomat.

Calling for the situation in Syria to be referred to the International Criminal Court for alleged war crimes to be investigated, he said: "As we speak, hundreds of thousands of people are trapped in 17 other besieged locations, and face life-threatening shortages of food, medicine and basic supplies."

"Armed opposition groups continue to fire mortars and other projectiles into civilian neighbourhoods of western Aleppo, but indiscriminate air strikes across the eastern part of the city by government forces and their allies are responsible for the overwhelming majority of civilian casualties," Zeid said.

Medical evacuations had been planned by the UN and the Red Cross on Friday to take advantage of a Russian ceasefire in which it also said it had opened up humanitarian corridors for civilians to leave rebel-held areas.

The Syrian government also dropped leaflets over east Aleppo to inform residents about the corridors.

But Jens Laerke, a spokesman for the United Nations humanitarian office, said: "Medical evacuations of sick and injured could unfortunately not begin this morning as planned because the necessary conditions were not in place."

"This is an astronomically difficult operation," he added.

The hour-long session at the Human Rights Council in Geneva, Switzerland, was called by the US and the UK to put pressure on Russia, Syrian President Bashar al-Assad's key ally, over its role in the Syrian war.

Speaking via video link, Tobias Elwood, the UK's minister for North Africa and the Middle East, said: “Russia, you are making the situation worse, not solving.”

Deputy US ambassador Ted Allegra said that the recent Russian and Syrian assault on rebel-held areas had killed 400 people, including 100 children.

"These shocking acts in Aleppo beg for an appropriate investigation and those who commit them must be held accountable," he said.

Ukraine's ambassador Yurii Klymenko also criticised Russia, which in 2014 annexed the Crimea region of Ukraine amid clashes between pro-Kiev and pro-Moscow forces east of the former Soviet country.

"We are witnessing Aleppo turning into another Grozny," said Klymenko, referring to the Chechen capital levelled by Russia in the 1990s.

The Guardian this week also revealed that Iran had personally requested Egypt to be in attendance at the Syria talks, a move commentators fear signifies Cairo veering from its pro-western stance in the region.

Both Russia and Syria hit back at the criticism directed towards them at the UN special session.

'The ancient city of Aleppo, a place of millennial civility and beauty, is today a slaughterhouse – a gruesome locus of pain and fear, where the lifeless bodies of small children are trapped under streets of rubble and pregnant women deliberately bombed'

Zeid Raad al-Hussein, UN Human Rights Council

Syria's ambassador Hussam Aala accused Western and Gulf countries of launching a "propaganda campaign" against his country.

Russian Ambassador Alexey Borodavkin accused Britain and its allies of "trying to save terrorists from being the target of strikes, allowing them to regroup and continue their barbaric acts".

An 11-hour unilateral ceasefire in Aleppo was "allowing civilians and those fighters who lay down their weapons to leave" the city, Moscow's envoy said.

Paulo Pinheiro, chairman of the UN commission of inquiry on Syria, said that the panel would continue to document war crimes in Aleppo and urged the Syrian government to provide information on violations.

"Hospitals, markets, bakeries and water stations have all been targeted by aeroplanes flying overhead; many have been destroyed, amplifying the effect of the siege," Pinheiro said.

Russian Foreign Minister Sergei Lavrov on Friday said that he suspected the West intended to protect Jabhat Fateh al-Sham, al-Qaeda's former Syrian affiliate, in a bid to ensure Assad is toppled.

The Russian Foreign Ministry on Friday also summoned the Belgian ambassador in Moscow over allegations that Belgian jets killed civilians in air strikes near Aleppo.

Russia's Defence Ministry and its Foreign Ministry said that two Belgian F-16 jets killed six civilians after it bombed a settlement in Aleppo province on 18 October.

Belgium has denied the claims and said Russia had fabricated the evidence to falsely accuse its air force.

Belgian Defence Minister Steven Vandeput demanded Russia retract its statement in what he described as “a groundless and unsubstantiated allegation”.