The cut-off is a major challenge to the government's effort throughout the nearly 6-year-old civil war to keep the capital as insulated as possible.

The truce does not include the Islamic State group or al-Qaida's branch in Syria. Several previous ceasefires all collapsed, some of them in a matter of days.

"The week serves as both a time for prayer and action to highlight the contributions of immigrants and vulnerable populations coming to the United States."

Speaking from Baghdad on Dec. 14, Kevin Hartigan, Catholic Relief Services' regional director for the Middle East, Central Asia and Europe told America that his team was bracing for what may be unprecedented numbers of people in flight from the contested city.