The jihadist militants who have seized large swathes of Iraq and Syria are intent upon creating "a house of blood", the UN's new human rights chief said Monday.

In his maiden address to the UN Human Rights Council, Zeid Ra'ad Al Hussein lashed out at the Islamic State militant group, 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 Zeid, the first Muslim and Arab to serve as UN High Commissioner of Human Rights.

"It would be a harsh, mean-spirited house of blood," warned the career diplomat.

- 'Annihilation of humanity' -

"In the Takfiri mind... there is no love of neighbour, only annihilation to those Muslims, Christians, Jews and others -- altogether the rest of humanity -- who believe differently to them," Zeid said.

He urged the world to make halting the "increasingly conjoined conflicts in Iraq and Syria" an "immediate and urgent priority".

The scale of the group's "use of brute violence against ethnic and religious groups is unprecedented in recent times," he said, warning that such attacks may constitute "a crime against humanity, for which those responsible must be held accountable."

Zeid'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.

Beyond the jihadist threat, the new human rights chief listed a range of other topics to be addressed during the three-week council session.

UN-mandated investigators would present their latest report on the situation in civil-war-ravaged Syria, he said, bringing "fresh evidence that this ancient civilisation has devolved into a slaughterhouse, where children are tortured in front of their parents or executed in public amid wanton killing and destruction."

Zeid also lashed out at Israel, stressing the need "to end persistent discrimination and impunity" in Gaza, where some 2,140 people were killed during the latest conflict.

"Current and future generations of Palestinians... have a right to live normal lives in dignity: without conflict, without a blockade, indeed without the wide range of daily human rights infringements that are generated by military occupation," he said.

Israel's "seven-year blockade must end," he said, also insisting on the right of Israelis to live "free and secure from indiscriminate rocket fire."

Zeid also highlighted the plight of migrants around the world, pointing to the nearly 1,900 who have died trying to cross the Mediterranean so far this year.

"More must be done by the EU and its member states to deal with this tragic situation," he insisted.

He also charged that Australia's practice of turning back vessels carrying migrants was "leading to a chain of human rights violations, including arbitrary detention and possible torture following return to home countries."

And he expressed concern over reports that the United States had detained some of the more than 50,000 unaccompanied children who have arrived in the past year, "fleeing violence and deprivation" in places like El Salvador, Guatemala and Honduras.

"Human rights are not reserved for citizens only, or for people with visas," Zeid said.