Gunmen of the so-called ’Islamic State of Iraq and the Levant’ (ISIL) took over Iraq’s largest Iraqi Christian town and surrounding areas on Thursday.

Gunmen of the so-called 'Islamic State of Iraq and the Levant' (ISIL) took over Iraq's largest Iraqi Christian town and surrounding areas on Thursday, causing tens of thousands of panicked residents to flee towards autonomous Kurdistan.

ISIL 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, witnesses 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 Agence France Presse.

"Tens of thousands of terrified people are being displaced as we speak, it cannot be described," the archbishop said.

Qaraqosh is an entirely Christian town which lies between Mosul and Arbil, the Kurdish region's capital. It usually has a population of around 50,000.

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.

Tal Kayf, the home of a significant Christian community as well as members of the Shabak Shiite minority, also emptied overnight.

"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.

ISIL, which proclaimed a "caliphate" straddling Syria and Iraq in late June, boasted of its latest occupation in a statement issued by its office in Mosul's Nineveh province.

"We are pleased to announce to the Islamic nation a new liberation in Nineveh province, teaching the secular Kurds a lesson," the statement said.

ISIL launched a devastating offensive in northwestern Iraq on June 9, seizing the country's second city Mosul the next day and sweeping across much of the northern region.