Fighters have taken over Iraq's largest Christian town Qaraqosh and surrounding areas and sent tens of thousands of panicked residents fleeing towards autonomous Kurdistan, officials and witnesses have said.
The Islamic State group moved in overnight after the withdrawal of Kurdish peshmerga troops, who are stretched thin across several fronts in Iraq, residents said.
Qaraqosh is an entirely Christian town which lies between Mosul, the Islamic State group's main hub in Iraq, and Erbil, the Kurdish region's capital. It usually has a population of around 50,000.
The Islamic State, which has declared a caliphate in parts of Iraq and Syria it controls, clashed with Kurdish forces on Wednesday in the town of Makhmour near Erbil. An Al Jazeera source however said peshmerga forces remained in control of Makhmour.
In the latest advance, fighters seized the mainly Christian town of Tilkaif, as well as al-Kwair, witnesses said.
"It's a catastrophe, a tragic situation. We call on the UN Security Council to immediately intervene. Tens of thousands of terrified people are being displaced as we speak, it cannot be described," Joseph Thomas, the Chaldean archbishop of Kirkuk and Sulaimaniyah, told the AFP news agency.
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Tilkaif, the home of a significant Christian community as well as members of the Shabak Shia minority, also emptied overnight.
"Tilkaif is now in the hands of the Islamic State. They faced no resistance and rolled in just after midnight," said Boutros Sargon, a resident who fled the town.
"I heard some gunshots last night and when I looked outside, I saw a military convoy from the Islamic State. They were shouting 'Allahu Akbar' [God is greatest]," he said.
In July, hundreds of families left Mosul in northern Iraq after the Islamic State issued an ultimatum to Christians to either convert or face death.
On Thursday, Pope Francis issued an "urgent appeal" to the international community to help end the crisis forcing residents to flee Iraq. He called on the international community "to take action to end the humanitarian tragedy now underway".