In his maiden address to the UN Human Rights Council, Zeid Ra'ad Al Hussein lashed out at the militant group calling itself the Islamic State, which has carved out a stronghold and declared a "caliphate" in an area straddling the border of the two conflict-torn nations.
"The Takfiris (extremists) who recently murdered (US journalist) James Foley and hundreds of other defenceless victims in Iraq and Syria, do they believe they are acting courageously, barbarically slaughtering captives?" the Jordanian prince told the opening of the council's 27th session in Geneva.
The massacres, beheadings, rape and torture attributed to IS militants "reveal only what a Takfiri state would look like, should this movement actually try to govern in the future," said Zaid, the first Muslim and Arab to serve as UN High Commissioner of Human Rights.
"It would be a harsh, mean-spirited house of blood, where no shade would be offered, nor shelter given to any non-Takfiri in their midst," warned the career diplomat.
He urged the world to make halting the "increasingly conjoined conflicts in Iraq and Syria" an "immediate and urgent priority."
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IS "has demonstrated absolute and deliberate disregard for human rights," Zaid said, stressing that "the scale of its use of brute violence against ethnic and religious groups is unprecedented in recent times."
He warned that attacks by the group motivated by ethnic background or religious beliefs may constitute "a crime against humanity, for which those responsible must be held accountable."
Zaid's speech to the UN's 47-member council came a week after it held an emergency session on the jihadists, deciding to send a fact-finding mission to Iraq to document the extent of their abuses.