As the Security Council met for emergency talks on the fighting in New York, Syria's opposition urged the UN to take immediate steps to protect civilians.
A Syrian government offensive to retake all of the divided city has battered Aleppo in recent days, with regime shelling on an opposition-controlled area reported to have killed at least 26 civilians.
Artillery shells rained down on one southeastern rebel-held district. The motionless body of a girl lay crumpled in the street, her arm severed and her head pierced by shrapnel. Rescue volunteers carried her body away on a motorcycle.
Speaking to a special Security Council session by video-link from London, Stephen O'Brien, the UN under-secretary-general for humanitarian affairs, appealed for action to stop the fighting.
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"For the sake of humanity we call on -- we plead -- with the parties and those with influence to do everything in their power to protect civilians and enable access to the besieged part of eastern Aleppo before it becomes one giant graveyard," he said.
Civilians have poured out of the besieged rebel-held east, battered by air strikes and heavy artillery fire by the advancing forces of President Bashar al-Assad.
Government troops and allied fighters have seized a third of the rebel-held east of Aleppo since they began an operation to recapture all of the city just over a fortnight ago.
They now fully control the city's northeast and pressed their offensive Wednesday on Aleppo's southeastern edges, advancing in the Sheikh Saeed district, according to state news agency SANA.
The loss of Aleppo would be the biggest blow for Syria's opposition since the conflict began in March 2011 with anti-government protests, before spiralling into a civil war.
WATCH: Aid agencies say 20,000 people have fled the fighting in eastern Aleppo
'Barbaric offensive'
The Syrian Observatory for Human Rights, a monitoring group, said Wednesday that more than 50,000 people have fled Aleppo's rebel-held districts, including at least 20,000 to government-held territory and another 30,000 to Kurdish-controlled districts.
Many others have travelled south into the remaining territory held by rebels.
The UN has for months sought access to the east, but a plan it presented earlier this month to deliver aid has yet to be approved by the government.
The Security Council session in New York was to later hear from the UN's peace envoy Staffan de Mistura.
Syria's opposition National Coalition said it was working with France on a draft UN resolution seeking an immediate ceasefire in Aleppo, though regime ally Russia was likely to veto such a proposal.
Coalition head Anas al-Abdeh on Wednesday urged the UN to "take immediate, definitive steps to protect civilians in Aleppo and stop the barbaric offensive against them."
"This escalation is a war crime and a crime against humanity, another item on the Assad regime's long, bloody, barbaric list," Abdeh said.
A source close to Syria's rebel factions said opposition forces held talks as recently as Monday with Russian representatives in Ankara on a potential truce for Aleppo.
'Waiting for death'
On Wednesday, at least 26 civilians including seven children were killed in government artillery fire on the Jubb al-Qubbeh district in east Aleppo, the Observatory said.
The White Helmets rescue group published pictures of the attack's aftermath, showing an apocalyptic scene with bodies, pieces of flesh, and suitcases strewn across the street.
A young man wept next to two bodies, their top halves obscured by blankets. The feet of one body were clad in pink socks, the other wore red boots done up with white laces.
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The Observatory says more than 300 civilians, including 33 children, have been killed in east Aleppo since the latest government assault began on November 15.
Another 48 civilians have been killed in west Aleppo, according to the monitor.
State news agency SANA said eight civilians including two children had been killed in rebel fire on several districts in the city's west on Wednesday.
The violence in the city has prompted international concern, though there has been little sign of a plan to intervene.
UN rights chief Zeid Ra'ad Al-Hussein on Wednesday said civilians in eastern Aleppo faced "a nightmare which clearly violates the most basic norms of human rights and any shred of human decency."
"Pounded by accelerating bombardment, deliberately deprived of food and medical care, many of them -- including small children -- report that they are simply waiting for death," he said.
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