It's reported Islamist militants have taken control of Qaraqosh, the largest Christian town in Iraq.
Thousands of believers are having to flee the city to escape ISIS fighters.
It's thought more than 96% of the population are members of the Syriac Catholic Church.
Jihadists stormed the town overnight forcing Kurdish troops to leave and Christians to flee to safety.
Last month ISIS issued an ultimatum to Christians in other parts of Iraq to convert to Islam, pay a tax or face death.
Joseph Thomas, the Chaldean archbishop of Kirkuk and Sulaimaniyah, told AFP: "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.
"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."
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Christian Solidarity Worldwide Special Ambassador Stuart Windsor told Premier: "We're deeply saddened by this news.
"There seems to be no end to this. Christians have fled the areas and the fact another has fallen is really so sad.
"We're urging Christians everywhere to pray for Iraq and we pray that this cleansing will come to an end and that we can see the international community do something about this terrible situation."
The AFP news agency is reporting an entire are in northern Iraq, home to a large part of the country's Christian community, has been taken over by ISIS.
Meanwhile as many as 40,000 people from religious minority groups are said to be stuck on top of a mountain as they tried to escape to Kurdistan.
Chaldean Catholic Patriarch Louis Raphael I Sako has written a prayer for Iraq:
"Lord,
The plight of our country
is deep and the suffering of Christians
is severe and frightening.
Therefore, we ask you Lord
to spare our lives, and to grant us patience,
and courage to continue our witness of Christian values
with trust and hope.
Lord, peace is the foundation of life;
Grant us the peace and stability that will enable us
to live with each other without fear and anxiety,
and with dignity and joy.
Glory be to you forever."