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."

'Perverse ideology'

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."

(BBC News)