Iraq Christians flee as Islamic State takes Qaraqosh
An international Christian organisation said at least a quarter of Iraq's Christians were leaving Qaraqosh and other surrounding towns.
IS has seized large parts of Iraq and Syria to create an Islamic caliphate.
Kurdish forces, known as the Peshmerga, have been fighting the Sunni militants' advance in the north for weeks.
In a separate development, the United Nations says it has rescued some of the thousands of people trapped by IS militants in mountains near the town of Sinjar.
Up to 50,000 members of the Yazidi religious minority fled there after IS overran Sinjar at the weekend.
Christian 'catastrophe' The French organisation Fraternite en Irak said on its Facebook page (in French) that a majority of inhabitants of Nineveh escaped when the militants took over Qaraqosh and surrounding towns.
As many as 100,000 people are believed to be fleeing toward the autonomous Kurdistan Region.
According to Fraternite en Irak, the commander of the Peshmerga in Qaraqosh told the town's archbishop late on Wednesday that the forces were abandoning their posts.
Several senior clergymen in Nineveh have now confirmed that the towns have fallen.
"It's a catastrophe, a tragic situation: tens of thousands of terrified people are being displaced as we speak," said Joseph Thomas, the Chaldean archbishop of the northern city of Kirkuk.
Eyewitnesses in Qaraqosh said IS militants were taking down crosses in churches and burning religious manuscripts.
The town - referred to as Iraq's Christian capital - is located 30km (19 miles) southeast of the city of Mosul, which was captured by IS in June.
Last month, hundreds of Christian families fled Mosul after the Islamist rebels gave them an ultimatum to convert to Islam or face death.
Iraq is home to one of the world's most ancient Christian communities, but numbers have dwindled amid growing sectarian violence since the US-led invasion in 2003.