Jihadists took over Iraq's largest Christian town and surrounding areas on Thursday and sent tens of thousands of panicked residents fleeing towards autonomous Kurdistan, officials and witnesses said.
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.
"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.
The number of people forced from their homes by the latest jihadist advance dwarfs the exodus sparked last month by an IS ultimatum to the Christians in Mosul.
"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.
The jihadist group 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 Sunni heartland.
The experienced peshmerga were initially thought to be a sufficient bulwark against massive further advances by the jihadists, but IS fighters have been moving stealthily across the northern Nineveh province and making surprise gains.
At the weekend, its units took over most of the Mosul hinterland which the peshmerga had occupied after government forces retreated in disarray two months ago.
Among the IS conquests was the Sinjar area, from which tens of thousands of civilians fled, including many from the Yazidi minority who are still hiding in nearby mountains.
Those Yazidi families, and other local residents, have been stranded in the barren Sinjar range since Saturday with little food and water.
Their plight has sparked international concern, while the leaders of the small minority, who practise a 4,000-year-old faith rooted in Zoroastrianism, have warned that their entire community risked being massacred or starved into extinction.
The Islamic State, which proclaimed a "caliphate" straddling Syria and Iraq in late June, boasted of its latest victories 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.
The group denied reports that an unprecedented alliance of Kurdish fighters from Iraq, Syria and Turkey was clawing back lost territory around Sinjar and elsewhere.