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"I am deeply alarmed by the ferocious pummelling of eastern Aleppo city," Mr O'Brien, UN under-secretary for humanitarian affairs, said in a statement, BBC News reports.

"Indiscriminate bombing and shelling continues in a shocking and unrelenting manner, killing and maiming civilians, subjecting them to a level of savagery that no human should have to endure."

Read alsoU.S. weighs tougher response to Russia over Syria crisis: officials – ReutersThe healthcare system in eastern Aleppo had been "all but obliterated", he added.

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Stephen O'Brien urged the warring parties to allow the evacuation of hundreds of people who need urgent medical care.

Rebel-held front-line areas were subject to dozens of Russian air strikes overnight, observers said.