The Islamic State (IS) group is carrying out genocide against the Christian, Yazidi and Shia minorities on its territory in Iraq and Syria, the United States declared Thursday.

"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 IS group.

Washington's senior diplomat made the ruling after the US Congress voted to designate the militant group's murders of religious minorities genocidal, a move which has international legal implications.

The United States is already leading a coalition of Western and Arab allies to strike IS group targets from the air and to support Iraqi government and Kurdish and Syrian militia forces against the group.