The World Food Programme (WFP) said the situation in the governorates of Najaf, Kerbala and Babel was reaching "critical levels" because of an influx of people who have fled violence in other parts of the country and no longer have any means of supporting themselves.
The Rome-based WFP said it was assisting 50,000 displaced families in Basra, Thi Qar, Qadissiya, Missan, Wassit, Muthanna, Najaf, Kerbala, and Babel.
Many are living in unoccupied public buildings or mosques and have already used up what savings they had getting to the relatively safe south from other, more expensive, parts of the country.
"The people receiving the aid are entirely dependent on it and are becoming vulnerable," WFP spokeswoman Abeer Etefa told AFP.
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"While the situation is now under control because we are feeding people, we are concerned that this assistance is not going to be sustainable once funding dries up in March and then these people will have to be on their own with the absence of any other form of regular assistance for food and other basic humanitarian needs."
The WFP warning was issued after a recent assessment of the situation which found many families were unable to say where their next meal would come from.
One of the people interviewed for the research was Najat Hussein, 36, a mother of six who said she had lost her husband seven months ago in the conflict in Tel Afar and subsequently moved to Kerbala.
"Time has stopped for us. There is no work, no schools and no future," she was quoted as saying by WFP. "We receive WFP food rations every month. Without this help I would be begging for food."
WFP aid involves the delivery of staples such as flour, rice, oil and pasta to families who have found semi-permanent places of refuge, emergency supplies to people still on the move and the issuing of food vouchers in areas where this can support local economies. The agency said it provided food support to 1.4 million people in Iraq in 2014.