Christians in the Iraqi city of Mosul were wrong-footed by its new jihadist masters, who initially left them in relative peace but later forced them to flee for their lives.

The turnaround in the attitude of Islamic State insurgents who overran Mosul last month could indicate the group is now confident enough of its hold on Iraq's second city to impose its extreme rules.

Analysts say the relative leniency the group had shown Christians may have been a sop to allied Sunni militant groups with a less severe interpretation of Islam.

"They tricked us, because in the beginning they did not threaten us, but after they established themselves they began imposing their terrorist laws on us," said Father Emmanuel Kelou, who once headed a Mosul church but now ministers to displaced Christians in the town of Qaraqosh, around 30 kilometres (20 miles) away.

Some Christians even returned to their homes after IS insurgents first took control of the city in a blistering offensive on June 9, lulled into a false sense of security by the near absence of attacks on co-religionists who stayed behind.

Last Monday, two nuns and three orphans were released in Mosul after being held for 17 days, a development the city's Chaldean Christian Patriarch Louis Sako described as "a glimmer of hope, and a breakthrough" in relations with the city's Islamist leaders.

Yet, days later and just over a month after arriving, insurgents would circulate a statement demanding Mosul's thousands of remaining Christians convert to Islam, pay a special tax or leave, triggering an exodus of people who had barely enough time to gather a few belongings.

On Friday the city's mosques were blaring calls for all Christians to depart by a deadline set for the following day, echoing the IS ultimatum seen by AFP, which said there would be "nothing but the sword" for those who did not comply.

Observers say IS's expulsion of Christians is in keeping with its proclaimed aim to create an Islamic caliphate in lands it has conquered, but that it may have waited to consolidate its hold on Mosul and other areas it had won before making a move.

The group, which espouses a puritanical version of Sunni Islam, swept vast swathes of northern and western Iraq in the days following the fall of Mosul, and is currently battling Shiite-led government forces in Saddam Hussein's former hometown of Tikrit and in western Anbar province.

- Moderate behaviour? -

"My expectation is that occupying a city while a military offensive is still ongoing is a complex task, and weeding out the Christians is not necessarily the most important thing to do first," said Jessica Lewis, a former US army intelligence officer who is now research director at the Institute for the Study of War.

Also, the Christians' treatment by IS, though harsh, was somewhat lenient for an organisation infamous for its mass executions, crucifixions and bloody videos and photos posted online. Shiite minorities in Iraq's north have been treated far worse.

The group may be trying to avoid alienating other Sunni militant factions who may not have any quarrel with Iraq's Christians, who have been an integral part of the country's social fabric for generations.

Elements of Saddam Hussein's disbanded socialist Baath party, for example, which once counted Christian deputy premier Tareq Aziz among their number, are fighting government forces alongside IS and could be a moderating influence on the Islamists.

"Considering the group's behaviour elsewhere and in the past, their option for Christians to flee could be seen as a slight moderation," said Charles Lister, a visiting fellow at the Brookings Doha Center.

"The IS will always be a severe and absolutist organisation, but considering the dynamics within the wider Sunni uprising in Iraq, it makes sense that it will be making small compromises for the sake of avoiding engendering unnecessary social tensions."

Still, it remains to be seen how far IS will go in imposing its austere brand of Islamic law now that it apparently feels more confident in areas that it controls.

The brutal application of sharia law by Al Qaeda-allied militants in Iraq in 2006 and 2007 turned many of their allies into foes as Sunni tribes rose up to eject Islamists in their midst.

"It will be interesting to see how far they will be able to push their social controls without eliciting a backlash from the local population and other insurgent groups," said Fanar Haddad, a research fellow at the Middle East Institute of the National University of Singapore.