Britain's ambassador to the United Nations has walked out of an emergency session of the Security Council after accusing Russia of committing war crimes.
Matthew Rycroft, alongside the French and US ambassadors, walked out of the sesson when Syria's ambassador was called to speak at an emergency meeting demanding a halt to the violence in Aleppo.
The walkout on Sunday by Mr Rycroft, US envoy Samantha Power, and France's Francois Delattre demonstrated Western anger and frustration at Syrian President Bashar Assad's decision to try to take Aleppo after US-Russian talks failed to revive a ceasefire.
In separate speeches to the council before the walkout, the three countries accused Russia of backing the Syrian offensive while talking about a cessation of hostilities.
Mr Rycroft said it was "difficult to deny" that both the Syrian regime and its Russian ally were committing war crimes.
He said: "After five years of conflict, you might think that the regime has had its fill of barbarity - that its sick bloodlust against its own people has finally run its course.
"But this weekend, the regime and Russia have instead plunged to new depths and unleashed a new hell on Aleppo."
The UN's special envoy on Syria, Staffan de Mistura, said a fresh offensive take the corner of the city still controlled by rebels had unleashed "unprecendented military violence" on its residents and killed at least 213 people - many of them women and children.
Mr de Mistura said there had been reports of bunker-buster bombs and incendiary weapons which created "fireballs of such intensity that they light up the pitch darkness in Aleppo as if it were daylight" being used.
Ms Power accused Russia of "barbarism" and said the US knew they had "consistently said one thing, and done the opposite."
She said Security Council members must "have the courage to say who is responsible for this and in a single voice tell Russia to stop".
But she added the US also believes it must do "everything in our power to find a way to halt the violence" and will "continue to look for any way possible to restore the cessation of hostilities."
The three days of relentless bombing meant many bodies were buried under the rubble of their destroyed homes and hospitals were overwhelmed with the wounded.
Meanwhile the water supply to an estimated two million people was cut off following a strike on a pumping station.
Additional reporting by agencies
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