Islamic State militants seized Iraq’s largest Christian town and surrounding areas Thursday, causing tens of thousands of people to flee, according to multiple news reports.

“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. ”It’s a catastrophe, a tragic situation. We call on the U.N. Security Council to immediately intervene. Tens of thousands of terrified people are being displaced as we speak, it cannot be described.”

Islamic State Seizes Iraqs Largest Christian Town: Its a Catastrophe

Iraqi Shiite volunteers who have joined government forces to fight Sunni jihadists from the Islamic State take part in a training session near the southern port city of Basra, August 7, 2014. (AFP/Getty Images/Haidar Mohammed Ali)

Qaraqosh, with a population of about 50,000, was an entirely Christian town about 19 miles from Mosul, which the jihadists formerly known as ISIS have made their stronghold after capturing it in June. The Islamic State has threatened Christians to either convert to Islam or be killed as part of the militants’ declared establishment of a caliphate across Iraq, Syria and the rest of the region.

“This is one of the biggest tragedies for Iraq’s Christians since 2003,” Faraj Benoit Camurat, the head of a Paris-based organization supporting Iraqi Christians, told AFP.

The Islamic State — comprised of Sunni Muslims — was able to overrun Qaraqosh in Nineveh province after Kurdish forces, which have been attempting to protect their region from the militants, withdrew, the BBC reported.

Islamic State Seizes Iraqs Largest Christian Town: Its a Catastrophe

Iraqi Shiite volunteers who have joined government forces to fight Sunni jihadists from the Islamic State take part in a training session near the southern port city of Basra, August 7, 2014. (AFP/Getty Images/Haidar Mohammed Ali)

“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,’” Boutros Sargon, a resident of Tal Kayf, told AFP. He said his town, home to another large Christian population, is “now in the hands of the Islamic State. They faced no resistance and rolled in just after midnight.”

The Islamic State on Thursday also seized Iraq’s largest dam, gaining control of a huge source of power and water resources, the Associated Press reported. The capture of the Mosul Dam came after a week’s worth of attempts, according to the AP.

In an online statement attributed to the Islamic State but not immediately verified, the militants vowed to continue “the march in all directions,” and said they would not “give up the great caliphate project.”