"Daesh is genocidal by self-proclamation, by ideology and by actions, in what it says, what it believes and what it does," Secretary of State John Kerry said.
"Daesh is also responsible for crimes against humanity against these same groups," he added, using the US government's preferred term for the group.
Washington's senior diplomat made the ruling after the US Congress voted to designate the jihadist group's murders of religious minorities genocidal, a move which has international legal implications.
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Nuri Kino, founder of the organization A Demand for Action, which works to protect minorities in Syria and Iraq, welcomed the decision in a Facebook post with of video clip of Kerry's statement:
The United States is already leading a coalition of Western and Arab allies to strike IS targets from the air and to support Iraqi government and Kurdish and Syrian militia forces against the group.