The United States declared that the Islamic State group's slaughter of Christians, Yazidis and Shiites in Iraq and Syria amounts to a genocide and vowed to halt it.
Secretary of State John Kerry's "moral statement" does not place the United States under any new legal obligations, but the White House said it could back an international investigation.
"The United States will cooperate with independent efforts to investigate genocide," President Barack Obama's spokesman Josh Earnest said.
Washington does not recognize the International Criminal Court, but officials said US agencies will collect evidence and work with international partners as states seek a way to bring justice to bear.
"Daesh is genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes and what it does," Kerry said, using a term for the IS group based on its Arabic acronym.
"Daesh is also responsible for crimes against humanity and ethnic cleansing directed at these same groups," he added.
The Islamic State group recruits Sunni extremists and has regularly carried out mass killings of Shiite Muslim, Christian and Yazidi prisoners.
In June 2014, it seized the formerly cosmopolitan city of Mosul in northern Iraq, placing whole communities under threat of murder, rape or enslavement.
Already in March last year, UN investigators warned the self-proclaimed caliphate was trying to wipe out Yazidis, members of a pre-Islamic religious minority.
The Simon Wiesenthal Center, which memorializes past genocides and campaigns against bigotry, welcomed the decision to name Christians and Yazidis as victims.
"We reiterate our call that the US put these two groups at the front of the line for consideration for immigration to our country," it said.
A State Department official, speaking on condition of anonymity, said the genocide ruling would not itself change the rules for granting refugee status.
But he insisted Syrian asylum seekers are already being considering sympathetically because the IS group's atrocities are long-known.
And, Kerry argued, the United States is already doing its utmost to halt the slaughter by leading a coalition to "degrade and destroy" the group.
Through air strikes and support for local forces, the coalition has pushed IS from 40 percent of the ground it once held in Iraq and 20 percent in Syria, he said.
"And currently we are engaged in a diplomatic initiative aimed at trying to end the war in Syria," he added.
Kerry argues Bashar al-Assad's brutal campaign to cling to power in Syria fuels the chaos that allowed the IS group to seize the east of his country.
He vowed to continue pressing for a negotiated settlement to the broader civil war to allow world militaries to focus their fire on the extremists.
"In my judgment, Daesh is responsible for genocide against groups in areas under its control including Yazidis, Christians and Shia Muslims," he said.
"For those communities, the stakes in this campaign are utterly existential," he said.
'Face of evil'
Kerry issued his ruling after the US Congress voted to declare the killings a genocide and demanded the administration take a position.
Representative Vern Buchanan, of Florida, welcomed a decision he said was long overdue.
"ISIS is the face of evil and there is no room for equivocation. Their actions clearly constitute genocide," he said, citing reports of Iraqi and Syrian Christians being tortured and crucified.
But, after the announcement, Representative Chris Smith, chairman of the House committee that oversees global human rights, asked: "Now what?"
Washington, Smith argued, should now lobby for an international court -- like those set up to prosecute war crimes in Rwanda or Yugoslavia -- to be created.
"A Syria tribunal would hold not only the genociders of ISIS but all parties -- especially the war criminal Bashar al-Assad, who has barrel-bombed Syrian civilians and killed tens of thousands -- accountable for their horrific deeds," he said.
The State Department official would not be drawn on calls for a new court, but said the United States would work closely with fellow members of the UN Security Council.
Murder and slavery
The IS group's history of murdering journalists, aid workers and suspected "spies" has made it difficult to document its crimes in great detail.
But the group has itself issued propaganda videos showing the mass killing of prisoners.
And it has issued its own legal rulings pronouncing that captured non-Muslim women slaves may be raped by its fighters.
"We've not been able to compile a complete record, I think that's obvious on its face," Kerry admitted.
"But over the past months we have conducted a review of the vast amount of information gathered by the State Department, by the intelligence community, by outside groups."