The US says the Islamic State (IS) group has committed genocide against Yazidis, Christians and Shia Muslims.
US Secretary of State John Kerry said IS was “genocidal by self-proclamation, by ideology and by actions”.
He did not say whether his declaration would lead to a change of US policy in the Middle East.
But he did call for an independent international investigation and criminal charges for those thought to be responsible for the atrocities.
“Naming these crimes is important,” Mr Kerry said, “but what is essential is to stop them.”
Mr Kerry also said IS was responsible for crimes against humanity and ethnic cleansing in areas it controls in Syria and Iraq.
He said his conclusions had been based on a wealth of evidence provided by the State Department, intelligence teams and other sources.
He also highlighted the execution of Christians in northern Iraq and Libya and the killing of Shia Turkmen in Iraq.
“The fact is that Daesh kills Christians because they are Christians, Yazidis because they are Yazidis, Shia because they are Shia,” he said, using an Arabic acronym for the group.
“This is the message it conveys to children under its control. Its entire world view is based on eliminating those who do not subscribe to its perverse ideology.”
What happened to the Yazidi community?
As IS militants advanced through northern Iraq in the summer of 2014 they attacked villages belonging to the Yazidis, who are regarded by the militant Islamists as infidels.
Hundreds of men and boys were killed and several thousands of young women were abducted as “spoils of war” and kept or sold on as sex slaves.
Tens of thousands of people were trapped for months on Mount Sinjar without food, water and shelter as they fled their attackers.
Who, What, Why: Who are the Yazidis?
Raped, beaten and sold: Yazidi women tell of IS abuse
Mr Kerry admitted that a lack of access to IS areas meant the US did not have a “complete picture” of the atrocities that had been carried out, and said he was “neither judge, nor prosecutor, nor jury”.
But he said he hoped its victims would take comfort in the fact that “the United States recognises and confirms the despicable nature of the crimes committed against them”.
It is only the second time the US administration has declared a genocide during a conflict. The previous time was in 2004 when then-Secretary of State Colin Powell used it to describe the killings in Darfur.
Mr Kerry had been given a congressional deadline of 17 March to announce whether IS’s actions in areas under its control constituted genocide.
Earlier this week, the US House of Representatives voted 393-0 to designate the crimes as genocide.
Some critics have accused the Obama administration of not speaking out forcefully enough about the treatment of minority groups by IS.
The administration has promised to do what it can to gather evidence for prosecutions, but it is far from clear what practical difference this declaration will make to persecuted groups in the Middle East, the BBC’s Gary O’Donoghue reports from Washington.
How the UN defines genocide
Article II of the 1948 UN Genocide Convention says genocide means any of the following acts committed “with intent to destroy, in whole or in part, a national, ethnical, racial or religious group”. They are:
- Killing members of the group.
- Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part.
- Imposing measures intended to prevent births within the group.
- Forcibly transferring children of the group to another group.