From Hamdi Alkhshali, CNN

Baghdad, Iraq (CNN) – Extremists occupying large swaths of Iraq and Syria have issued a threat to Iraqi Christians in the city of Mosul:

Accept Islam, pay extra taxes to Islamic Sharia courts or face “death by the sword.”

The letters from the Islamic State in Iraq and Syria, or ISIS, were distributed in recent days to leaders of the dwindling Christian minority in Iraq’s second largest city.

The message added that ISIS leader Abu Bakr al-Baghdadi has agreed to allow them (Christians who do not agree to convert or pay extra taxes) to leave the embattled city by noon Saturday (5 a.m. ET). After that, the message said, “the Militant ISIS jihadists, a Sunni-dominated al Qaeda splinter group, have overrun large parts of Iraq and neighboring war torn Syria over the past months in a violent Islamist insurgency. The militants want to establish an Islamic state, or so-called caliphate, across Sunni areas of both countries.

ISIS already controls hundreds of square miles where state authority has evaporated. It has ignored international borders, establishing a deadly presence from Syria’s Mediterranean coast all the way south to Baghdad, making its goal of a caliphate state seemingly within reach.

The magnitude of the crisis is clear from the sharp rise in the death toll over the past two months.

At least 2,400 Iraqis died in violence in June, according to the United Nations Assistance Mission for Iraq. Of those, the United Nations said more than 1,500 were civilians, including 270 civilian police officers, and almost 900 were members of Iraqi security forces.

U.S. President Barack Obama has authorized the deployment of 300 military advisers to Iraq. More than 200 are already there.

Obama has also ordered a review of Iraqi security forces to determine the best steps to take to help the embattled country. Many Iraqi troops abandoned their posts and fled when faced with attacks from ISIS, which contributed to the defenselessness of major parts of the country and helped fuel the insurgency.

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Shelby Lin Erdman contributed to this report