Islamic State (IS) militants moved in overnight after the withdrawal of Kurdish peshmerga troops, who are stretched thin across several fronts in Iraq, residents said.
"I now know that the towns of Qaraqosh, Tal Kayf, Bartella and Karamlesh have been emptied of their original population and are now under the control of the militants," Joseph Thomas, the Chaldean archbishop of Kirkuk and Sulaimaniyah, told AFP.
Qaraqosh is an entirely Christian town which lies between Mosul, the jihadists' main hub in Iraq, and Arbil, the Kurdish region's capital. It usually has a population of around 50,000.
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"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," the archbishop said.
Tal Kayf, the home of a significant Christian community as well as members of the Shabak Shiite minority, also emptied overnight.
"Tal Kayf 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 and was reached by phone in Arbil.
"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.
