An AFP correspondent in Mosul, the main Iraqi hub of the Islamic State (IS) group's proclaimed "caliphate", said Christians were squeezing into private cars and taxis to beat a noon deadline.
"Some families have had all their money and jewellery taken from them at an insurgent checkpoint as they fled the city," said Abu Rayan, a Mosul Christian who had just driven out with his family.
"Some of our homes have already been confiscated and I know families who have handed their keys to neighbours, asking them to look after their property with the hope they would return one day."
An earlier statement by Mosul's new rulers had said there would be "nothing for them but the sword" if Christians did not abide by those conditions by 0900 GMT today.
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While some families initially appeared prepared to pay the "jizya" Islamic tribute to stay in their ancient homes, messages broadcast my mosques yesterday appeared to spark an exodus.
"Christian families are on their way to Dohuk and Arbil" in autonomous Iraqi Kurdistan, Chaldean patriarch Louis Sako, who heads Iraq's largest Christian community, told AFP.
The Islamic State "seems intent on wiping out all traces of minority groups from areas it now controls in Iraq," Human Rights Watch said in a statement today.
Other minorities rooted in the same province of Nineveh have suffered even more, according to crimes which HRW documented against the Yazidis, as well as the Turkmen and Shabak Shiite communities.
The mass displacement was the latest in weeks of turmoil which has forced more than 600,000 people from their homes, left thousands dead and brought Iraq to the brink of collapse.