Islamic State (IS) militants moved into Qaraqosh and several other towns 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.
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.
"This is one of the biggest tragedies for Iraq's Christians since 2003," said Faraj Benoit Camurat, the Paris-based head of an association supporting Iraqi Christians and other minorities.
A peshmerga spokesman said Kurdish forces were battling the Islamic State in Qaraqosh and Al-Qosh further north, but no witnesses could corroborate that claim.
The spokesman also said the peshmerga were fighting in Gwer, a Kurdish community south of Qaraqosh.
The IS advance means jihadists are now within striking distance, in some areas barely 20 kilometres (12 miles) away, of the official border of the Kurdish Regional Government and 40 kilometres from Arbil.