Story highlights United Nations cancels planned medical evacuations from Aleppo

Official says attacks on civilian areas in eastern Aleppo constitute war crimes

(CNN) The top UN human rights official called the siege and bombardment of eastern Aleppo "crimes of historic proportions" on Friday, and accused all parties of violating international humanitarian law in Syria.

In a video speech to the UN Human Rights Council in Geneva, Switzerland, Zeid Ra'ad Al Hussein said armed opposition groups are firing mortars into neighborhoods of western Aleppo, but "indiscriminate airstrikes across the eastern part of the city by government forces and their allies are responsible for the overwhelming majority of civilian casualties."

He said these violations constitute "war crimes," and "if knowingly committed as part of a widespread or systematic attack directed against civilians, they constitute crimes against humanity."

The forces of Syrian President Bashar al-Assad, backed by Russian warplanes, have been pummeling eastern Aleppo as they seek to wrest it from rebel control.

However, a Russian-declared "humanitarian pause" began Thursday, which Russia said would allow rebels and civilians to leave rebel-held sections of the city during daytime hours for three days.

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