Thousands of Iraqi Christians are fleeing after Islamic State militants seized the minority's biggest town in Iraq, according to reports. The Islamic State, formerly ISIS, captured Qaraqosh in Nineveh province overnight after Kurdish forces withdrew from the town. The French Christian organisation Fraternite en Irak said in a post on Facebook that a majority of those living in Nineveh left after militants took over Qaraqosh, known as the Christian capital of northern Iraq, and surrounding towns. "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. "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 overrunning of Qaraqosh comes after Islamic State fighters pushed back Kurdish forces in north Iraq over the weekend. Reports suggest that the Sunni extremists now control the entire northern area of Iraq. According to Fraternite en Irak, the commander of the Kurdish troops in Qaraqosh told the town's archbishop late on Wednesday that they would be leaving. A number senior religious leaders in Nineveh are said to have confirmed that the towns have fallen. Eyewitnesses in Qaraqosh have said that Islamic State militants have been removing crosses in churches, according to the BBC.