Ban Ki-moon, the UN secretary-general, has condemned the persecution of Iraqi Christians by Islamic State (IS) fighters, saying it may constitute a crime against humanity.

The statement on Monday came after hundreds of Christian families fled their homes in the northern city of Mosul as an ultimatum threatening their community's centuries-old presence ended.

The fighters, who have run the city after a military offensive that began six weeks ago, had issued a written declaration to Mosul's Christian community to either convert, pay a tax, leave or face imminent execution.

Mosul's new rulers said there would be "nothing for them but the sword" if Christians did not abide by those conditions by Saturday.

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The Sunni-led Islamic State group issued the the ultimatum in a letter after Friday prayers.

The Islamic State document, obtained by Al Jazeera, states that the order was issued after Christian leaders failed to attend a meeting called by the group.

In response, the group says in the letter that Christians must either convert to Islam, pay a tax on non-Muslims known as "jiziya", or give up their posessions and leave the city.

Failure to do so would result in a death penalty, "as a last resort".

While some families initially appeared prepared to pay the Islamic tribute to stay in their homes, messages broadcast by mosques on Friday sparked an exodus.

Ban has stressed that "any systematic attack on the civilian population, or segments of the civilian population, because of their ethnic background, religious beliefs or faith may constitute a crime against humanity", adding that he was "particularly disturbed by reports of threats against Christians in Mosul and other IS-controlled parts of Iraq".

Mosul, Iraq's second largest city, was overrun by the Islamic State and allied rebel groups last month.

Prior to the 2003 US invasion, more than a million Christians lived in Iraq, including more than 600,000 in Baghdad and 60,000 in Mosul, as well as a substantial number in the oil city of Kirkuk and in Basra.

Until their forced exodus over the weekend, Christians had been continuously present in Mosul for approximately 16 centuries.

Source: Al Jazeera and agencies