The British ambassador to the United Nations walked out of the UN's Security Council meeting on Sunday (25 September) after a heated session in which Syria and Russia were accused of war crimes over the offensive in Aleppo.
Western anger boiled over when Matthew Rycroft, was joined by the French ambassador Francois Delattre and US envoy Samantha Power in leaving the session as Syria's ambassador was called to speak, Associated Press reported.
Before the dramatic walkout the three countries accused Russia of backing the Syrian offensive while talking about a cessation of hostilities.
Rycroft said it was "difficult to deny" that both the Syrian regime and its Russian ally were committing war crimes.
"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."
However Russia's UN Ambassador Vitaly Churkin said: "Bringing a peace is almost an impossible task now", blaming Syria's rebels for sabotaging the cease-fire agreement by shoring up its forces.
He accused the Western coalition of failing to separate the moderate forces it backs from "terrorist" groups including the al-Qaida-linked Fatah Sham Front.
He insisted that Russia still wants a cease-fire along with renewed negotiations between the Syrian parties.
After the end of a week-long cessation of hostilities on Monday (19 September), the Syrian regime declared a new offensive against eastern Aleppo and jets began pounding the area, where 275,000 people are trapped.
UN special envoy on Syria Staffan de Mistura said at least 213 civilians had been killed since the offensive began, while humanitarian workers on the ground reported that approximately half of the casualties were children.
He told the Security Council: "Since the unravelling of the cessation of hostilities and the unleashing of unprecedented military violence affecting civilians we have seen the situation in Aleppo reach new heights of horror."