By Gretel Johnston

Washington (dpa) - The Islamic State militant group is committing genocide against minority groups in Iraq and Syria, including Christians, Yazidis and Shia Muslims, US Secretary of State John Kerry said Thursday.

Islamic State "kills Christians because they are Christians, Yazidis because they are Yazidis, Shias because they are Shias," Kerry said at the State Department in Washington.

"Daesh is genocidal by self-proclamation, by ideology, and by actions - in what it says, what it believes, and what it does," Kerry said, referring to Islamic State by its Arabic acronym. "Its entire world view is based on eliminating those who do not subscribe to its perverse ideology."

He noted that the Sunni extremist militia has committed crimes against humanity and ethnic cleansing directed at these groups, and in some cases also against Sunni Muslims, Kurds and other minorities.

"Civil war fuels Daesh," the top US diplomat said, but acknowledged also that the 66-member international coalition to stop Islamic State had managed to pushed the extremists out of 40 per cent of the territory it once controlled in Iraq and 20 per cent in Syria.

"There is no question in my mind that if Daesh succeeded in establishing its so-called caliphate, it would seek to destroy what remains of ethnic and religious mosaic once thriving in the region," Kerry said.

The US Congress has been pressuring the Obama administration to report on its investigations into genocide and other atrocities in Iraq and Syria and had set Thursday as the deadline for releasing its findings.

Kerry said his conclusions were based on a review of information gathered over the past months by the State Department, the intelligence community and outside groups.

While saying the US would do all it can to see that the perpetrators are held accountable, it was unclear whether it would have an immediate impact on the military effort against the group.

Kerry said part of the US response must be to destroy it by military force, but he also mentioned the importance of emergency aid. His conclusion of genocide underscores the need for a comprehensive and unified approach to defeating Islamic State, he said.

"I am neither judge, nor prosecutor, nor jury with respect to the allegations of genocide, crimes against humanity, and ethnic cleansing by specific persons," he said, adding that in the end the facts "must be brought to light by an independent investigation."

Kerry listed numerous examples of Islamic State crimes directed against religious groups and others, including forcing Christian women and girls into sexual slavery.

It was unclear on Wednesday whether the State Department would meet the Thursday deadline set by Congress on its findings on a genocide designation.

But Kerry said he had in his judgement decided that genocide had occurred and the determination to act against it "must be pronounced among decent people all across the globe."

But the likelihood of any immediate action was downplayed Wednesday by State Department spokesman Mark Toner. He said acknowledging that genocide or crimes against humanity had taken place "would not necessarily result in particularly obligations for the United States."

He added that everyone agrees that what Islamic State is carrying out against the ethnic population and religious minorities needs to be stopped, and the US would continue putting pressure on the militant group "to try to strangle Daesh as quickly as possible."

House Foreign Affairs Committee Chairman Ed Royce welcomed Kerry's statement.

"Secretary Kerry is finally making the right call," Royce said in a statement. He urged President Barack Obama to lay out a "broad, overarching plan" to actually defeat and destroy Islamic State.