The United Nations's second most senior official in Iraq, Jacqueline Badcock, told reporters in Geneva via videoconference: "It is a fatwa (or religious edict) from ISIS, we learnt about it this morning".
"We have no precise numbers," she said on Thursday.
The Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIS), took over large swathes of the country last month and has begun imposing its extreme Salafist interpretation of Islam.
But several experts have speculated that the fatwa may have been a hoax, and a number of journalists said on Twitter that their contacts in Iraq had not heard of it being issued.
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