The so-called Islamic State militant group has committed genocide against minority Christians and Yazidis as well as Shi'ite Muslims, US Secretary of State John Kerry said.
"The fact is that Daesh kills Christians because they are Christians; Yazidis because they are Yazidis; Shi'ites because they are Shi'ites," Mr Kerry said, referring to the group by an Arabic acronym, and accusing it of crimes against humanity and of ethnic cleansing.
While the genocide finding may make it easier for the United States to argue for greater action against the group, it does not create a legal obligation on the United States to do more.
Yesterday, a US State Department spokesman Mark Toner said: "Acknowledging that genocide or crimes against humanity have taken place in another country would not necessarily result in any particular legal obligation for the United States."
IS militants have swept through Iraq and Syria in recent years, seizing control of large swathes of territory with an eye toward establishing jihadism in the heart of the Arab world.
The group's videos depict the violent deaths of people who stand in its way. Opponents have been beheaded, shot dead, blown up with fuses attached to their necks and drowned in cages lowered into swimming pools.
US President Barack Obama ordered air strikes against the group but has not made any large commitment of US troops on the ground.
"It may strengthen our hand getting other countries to help. It may free us against some (legal) constraints, but the reality is that when you are fighting somebody, you don't need another reason to fight them," said Jon Alterman, director of the Middle East program at the Centre for Strategic and International Studies.
Mr Kerry argued that the United States has done much to fight the group since the start of air strikes in 2014, but did not directly answer a question on why the Obama administration had not done more to prevent genocide.
Historians have asked the same question about Darfur and Rwanda, both places where the United States also concluded that genocide had taken place.
IS militants have exploited the five-year civil war in Syria to seize areas in that country and in neighbouring Iraq, though US officials say their air strikes have markedly reduced the amount of territory the group controls in both.
On-again, off-again peace talks got under way this week in Geneva in an effort to end the civil war, in which at least 250,000 people have died and millions have fled their homes. A fragile "cessation of hostilities" has reduced, but not ended, the violence over the last two weeks.
US lawmakers urged Mr Kerry last year to make a determination on whether atrocities committed by the militant group against Christians and other religious groups amounted to genocide.
Asked if the genocide finding was likely to accentuate a view held by some that the West is engaged in a battle with Islam, a senior US official noted that Mr Kerry's determination included Shi'ite Muslims as well as Christians and Yazidis.
"A very large Muslim group is included in this," said the official. "The secretary isn't just singling out one group that has political support in the United States," he added, referring to Christians.
