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Iraq's prime minister on Sunday condemned ISIS's actions targeting Christians in territory it controls, saying they reveal the threat the jihadists pose to the minority community's "centuries-old heritage."

The comments from Nouri al-Maliki come a day after the expiration of a deadline imposed by ISIS, which also goes by the name the Islamic State, calling on Christians in the militant-held city of Mosul to convert to Islam, pay a tax or face death. Most Christians opted to flee to the nearby self-rule Kurdish region or other areas protected by Kurdish security forces.

"What is being done by the Daesh terrorist gang against our Christian citizens in Ninevah province, and their aggression against the churches and houses of worship in the areas under their control reveals beyond any doubt the extremist criminal and terrorist nature of this group," al-Maliki said in a statement released by his office, using the Arabic acronym for the extremist group.

"Those people, through their crimes, are revealing their true identity and the false allegations made here and there about the existence of revolutionaries among their ranks."

At the Vatican, Pope Francis expressed his concern Sunday for Mosul's Christians, offering prayers in his traditional Angelus blessing for Iraqi Christians who "are persecuted, chased away, forced to leave their houses without out the possibility of taking anything" with them.

UN statement

A gunman watches an Iraqi police truck burn in front of the provincial government headquarters in Fallujah, Iraq, in January. (The Associated Press)

In New York, the office of UN Secretary-General Ban Ki-Moon issued a statement condemning ISIS attacks on all religions, not just Christians.

"The Secretary-General condemns in the strongest terms the systematic persecution of minority populations in Iraq by Islamic State (IS) and associated armed groups,"  the statement said. "He is particularly disturbed by reports of threats against Christians in Mosul and other IS-controlled parts of Iraq, including an ultimatum to either convert, pay a tax, leave, or face imminent execution. Equally repugnant are reports that Turkoman, Yazidis and Shabaks are facing abductions, killings or the destruction of their property, and that the homes of Christian, Shia and Shabak residents in Mosul have been marked."

And after noting that "minority communities that have lived together for thousands of years" in the region, the statement warned "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, for which those responsible must be held accountable."

Residents in Mosul also say ISIS's fighters recently have begun to occupy churches and seize the homes of Christians who have fled the city.

These actions stem from the harsh interpretation of Islamic law the group seeks to impose on the territory it controls in Iraq and neighbouring Syria. Already in Mosul, the extremist group has banned alcohol and water pipes, and painted over street advertisements showing women's faces. It has, however, held off on stricter punishments so far.

ISIS gunmen on patrol in Fallujah. (File/Associated Press)

Iraq's Christian communities date back to the first centuries of the religion. Before the 2003 U.S-led invasion, around 1 million Christians called Iraq home. But since then, the community has been a frequent target for militants, and attacks on churches, worshippers and clergymen has prompted many Christians to leave the country. Church officials now estimate the community at around 450,000.

The UN special representative to Iraq, Nickolay Mladenov, condemned the persecution of all minority religious communities, including Christian, Shabak, Yazidi and Turkmen, in Mosul and the surrounding province of Ninevah. Mladenov said the UN will provide humanitarian assistance to the displaced.

Under pressure to resign

Al-Maliki also tried to rally support for those uprooted from their homes, calling on government agencies to provide "all the necessities for a decent life" for them. He also took the opportunity to urge "the whole world to tighten the siege on those terrorists and stand as one force to confront them."

The prime minister, who has ruled the country since 2006, is under pressure to step aside and not seek a third consecutive term. Many in Iraq accuse al-Maliki's Shia-led government of helping fuel the crisis by failing to promote reconciliation with the Sunni Muslim minority, and say he has become too polarizing a figure to unite the country and face down the militant threat.

Armed men stand guard beside people in Najaf, south of Baghdad, shouting slogans in support of the call to arms by Grand Ayatollah Ali al-Sistani, Iraq's most senior Shi'ite Muslim cleric. Sistani urged followers to take up arms against Sunni militants. (REUTERS)

So far, Iraq's security forces have struggled to claw back any of the ground they have surrendered to the militants over the past five weeks. The only major counteroffensive has failed to make major headway in retaking the northern city of Tikrit.

The insurgents, meanwhile, have not made any further major gains since overrunning the majority of Iraq's predominantly Sunni areas. But they continue to attack government forces across several fronts, while also carrying out smaller scale attacks in Baghdad.

ISIS claimed responsibility late Saturday for four bombings in Baghdad which were among a string of attacks that killed at least 27 people earlier in the day. The violence was among the most significant in Baghdad since the Sunni militant offensive began.