The siege and bombing of eastern Aleppo in Syria constitute "crimes of historic proportions" that have caused heavy civilian casualties amounting to "war crimes", according to the top United Nations human rights official.

Zeid Ra'ad al Hussein's comments on Friday came during a special session of the UN human rights council called by Britain to set up a special inquiry into violations, especially in Aleppo's rebel-held east where an estimated 275,000 civilians are besieged by a Syrian government offensive backed by Russia. 

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In a video speech, Zeid said Aleppo is a "slaughterhouse" and called for major powers to put aside their differences and refer the situation in Syria to the prosecutor of the International Criminal Court (ICC).

"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 told the session.

The "collective failure of the international community to protect civilians and halt this bloodshed should haunt every one of us", he added.

Once Syria's largest city, Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012. 

Hundreds of civilians have been killed or wounded in Syrian and Russian air strikes since the collapse of the latest ceasefire and the announcement by President Bashar al-Assad's government last month of a major ground offensive to retake the city.

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Another 82 people have died in rebel fire on government-held neighbourhoods in the west, according to the UK-based Syrian Observatory for Human Rights.

Paulo Pinheiro, chairman of the UN commission of inquiry on Syria, also addressed the special session and said that the panel would continue to document war crimes in the northern city.

Pinheiro also appealed to the government to provide information on violations.

While rights council resolutions are non-binding, Russia is expected to push back against any draft strongly condemning Assad's government.

The session, also supported by France, Germany and the United States, as well as Turkey, is aiming to adopt a resolution later on Friday.

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Earlier this week, the European Union strongly condemned Russia and the Syrian government for causing "untold suffering" and suggested their actions in the city may amount to "war crimes".

"The deliberate targeting of hospitals, medical personnel, schools and essential infrastructure, as well as the use of barrel bombs, cluster bombs, and chemical weapons, constitute a catastrophic escalation of the conflict ... and may amount to war crimes," European Union foreign ministers said in a joint statement.

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On Monday, France announced that it would ask the ICC to investigate possible war crimes committed in Aleppo.

"France is committed as never before to saving the population of Aleppo," Jean-Marc Ayrault, the French foreign minister, told radio on Monday.

British Foreign Secretary Boris Johnson also gave his backing to French calls for an ICC investigation into alleged war crimes in the city. 

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The Syrian military said on Thursday that a unilateral ceasefire backed by Russia had come into force to allow people to leave besieged eastern Aleppo, but rebels rejected the move saying they are preparing a counter-offensive to break the blockade.

Al-Mayadeen TV aired live footage on Friday from Aleppo's Castello Road showing bulldozers clearing the area, as well as buses and ambulances parked on the roadside to take evacuees.

But residents in the besieged area said there were no guarantees that evacuees will not be arrested by government forces.

Rebels said the goal of Moscow and Assad is to empty opposition-held areas of civilians so they can take over the whole city.

Later on Friday, the UN said medical evacuations from eastern Aleppo had not begun on Friday as it had hoped, as a lack of security guarantees and "facilitation" prevent aid workers taking advantage of the pause in the bombing announced by Russia.

Source: Al Jazeera News And News Agencies