On Friday, the U.N. High Commissioner for Human Rights, Zeid Ra’ad al-Hussein of Jordan, called the siege and bombing of Aleppo by Russian and Syrian forces “crimes of historic proportions,” described the besieged city as a “slaughterhouse,” and called for prosecution by the International Criminal Court.

In a videotaped address, Zeid asked the U.N. Human Rights Council in Geneva to set aside its “political disagreements” and make a referral to the ICC. He said the “collective failure of the international community to protect civilians and halt this bloodshed should haunt every one of us.”

“Armed opposition groups continue to fire mortars and other projectiles into civilian 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,” Zeid charged, pointing to the regime of Bashar Assad and his Russian patrons as the primary offenders.

Zeid said the Aleppo attacks were “war crimes,” and if they were “knowingly committed as part of a widespread or systematic attack directed against civilians, they constitute crimes against humanity.”

He demanded an “immediate, prolonged and all-encompassing ceasefire” to allow relief efforts in Aleppo, saying that “all parties must provide assistance and free passage for all civilians wishing to flee, without any form of reprisal – including passage across international borders.”

CNN reports that, despite Russia’s announcement of a bombing pause for humanitarian relief, the United Nations has canceled its plans to begin medical evacuations from Aleppo.

“We cannot begin safe, secure and voluntary evacuation of the sick and critically wounded and families,” said U.N. Deputy Spokesman for Humanitarian Affairs Jens Laerke.

Laerke added that U.N. officials would continue to assess the situation in Aleppo on a “day-by-day, hour-by-hour basis,” and were still hopeful that humanitarian operations could begin after “all the pieces of the complicated puzzle fall into place.”

European Council President Donald Tusk also called for an “end to the atrocities and an immediate cessation of hostilities” from Brussels.

Tusk said the European Union “strongly condemned the attacks by the Syrian regime and its allies, notably Russia, on civilians in Aleppo,” and warned the EU will “consider all available options if these atrocities continue.”