Zeid Ra'ad al Hussein did not specifically name Russia, whose war planes have carried out weeks of air strikes on the rebel-held part of Aleppo along with the Syrian air force, but his reference was clear.
"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 said in a speech by video to a special session of the U.N. Human Rights Council.
He called for major powers to put aside their differences and refer the situation in Syria to the prosecutor of the International Criminal Court (ICC).
"The violations and abuses suffered by people across the country, including the siege and bombardment of eastern Aleppo, are simply not tragedies; they also constitute crimes of historic proportions," Zeid told the Geneva session.
Russia has denied any deliberate targeting of civilians and says it is combating terrorists.
Britain, which requested the one-day session along with allies including the United States in order to set up a special inquiry on Aleppo, sought to shame Russia for its air strikes on Aleppo.
"Russia, you are making the situation worse, not solving it," Tobias Ellwood, the British government minister for Africa and the Middle East, said in a speech to the Geneva forum.
"This is shameful and it is not the action or leadership that we expect from a P5 nation," he said, referring to the five permanent members of the U.N. Security Council -- Britain, China, France, Russia and the United States.
Russian Ambassador Alexey Borodavkin accused Britain and its allies of "trying to save terrorists from being the target of strikes, allowing them to regroup and continue their barbarbic acts".
An 11-hour unilateral ceasefire in Aleppo, offered by Russia on Thursday, was "allowing civilians and those fighters who lay down their weapons to leave" the city, Moscow's envoy said.
Paulo Pinheiro, chairman of the U.N. commission of inquiry on Syria, said that the panel would continue to document war crimes in Aleppo in an impartial way and urged the government of President Bashar al-Assad to provide information on violations.
"Over a quarter of a million people have been besieged and subjected to withering daily air strikes for more than one month," Pinheiro said. "Hospitals, markets, bakeries and water stations have all been targeted by airplanes flying overhead; many have been destroyed, amplifying the effect of the siege."
Syria's ambassador Hussam Aala accused Western and Gulf countries of launching a "propaganda campaign" against his country and said Syria rejects the "lies and manipulation of the Human Rights Council by Britian and its allies".