Christians who do not accept Islam or fail to pay extra taxes to the Islamic Sharia courts, the letters warned, will face "death by the sword."
Christians who do not agree to ISIS demands were also given the option to leave Mosul. According to a message from ISIS leader Abu Bakr al-Baghdadi, non-complying Christians were permitted to "evacuate themselves only from the caliphate state" by 12 p.m. local time on Saturday or their "only option is the sword."
The warning comes as ISIS has reportedly withdrawn from parts of Mosul, replaced by another Sunni militant group, the Naqshbandi Army (also referred to as JRTN). Mosul governor Atheel Nujaifi told the Guardian that the ISIS "strike force" had relocated from the city to fight Iraqi armed forces in Tikrit. JRTN, he said, now controls the eastern half of Mosul.
There is some speculation that JRTN, led by Saddam Hussein-era Baathists, formed an alliance with ISIS that enabled the Sunni militant lightning offensive in June to seize control of key Iraqi cities.