The UN Human Rights Council in Geneva has opened a special session on Syria's besieged city of Aleppo.
The session, called by the United Kingdom, is seeking a resolution on abuses especially in the northern city's rebel-held districts, where some 250,000 civilians are besieged by Syrian government forces backed by Russia.
Addressing the 47-member council, UN human rights chief Zeid Ra'ad al-Hussein demanded action against "crimes of historic proportions."
"Armed opposition groups continue to fire mortars and other projectiles into civilian neighborhoods of western Aleppo, but indiscriminate air strikes across the eastern part of the city by government forces and their allies are responsible for the overwhelming majority of civilian casualties," Zeid also said.
Tobias Ellwood, the British government minister for Africa and the Middle East, criticized Moscow for its "shameful" actions in Aleppo, saying, "Russia, you are making the situation worse, not solving it."
But Russian Ambassador Aleksei Borodavkin said a unilateral cease-fire in Aleppo was "allowing civilians and those fighters who lay down their weapons to leave" the city.
He also accused the United Kingdom and its allies of "trying to save terrorists from being the target of strikes, allowing them to regroup and continue their barbaric acts."