Zeid Ra'ad Al Hussein said in a statement that renewed violence in Syria, including strikes on markets and medical facilities, showed a "monstrous disregard for civilians lives by all parties to the conflict".
But he directed especially tough criticism towards the powerful countries influencing the conflict.
"The persistent failure of the Security Council to refer the situation in Syria to the International Criminal Court (ICC) is an example of the most shameful form of realpolitik," Zeid said.
"In the minds of many, the world's great powers have in effect become accomplices to the sacrifice of hundreds of thousands of human beings, and the displacement of millions."
The comments appeared to be reference to both the United States, which is backing some rebel groups and Russia, a key supporter of President Bashar al-Assad.
Assuming Damascus does not directly ask the ICC to intervene, referring the conflict to The Hague-based court would require approval of the Security Council's permanent members, including Russia and China which until now have blocked any such referral.
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The rights chief joined a list of world leaders who have expressed grave concern over the rapid deterioration of a ceasefire declared on February 27.
Through March, the fragile truce led to a significant decline in violence and increased access for humanitarian workers.
But fighting has surged over the last two weeks, putting added strain on UN-mediated peace talks in Geneva.
The main opposition High Negotiations Committee suspended its participation in the talks last week to protest at renewed government offensives.
"The violence is soaring back to the levels we saw prior to the cessation of hostilities," Zeid said, warning that all signs pointed to "a lethal escalation" in the conflict.
More than 200 civilians have been killed in Aleppo over the past week as rebels have pounded government-held neighbourhoods with rocket and artillery fire and the regime has hit rebel areas with air raids.
Fighting has also surged around Damascus, Homs and other areas.
Syria's conflict has left more than 270,000 people dead since 2011.