"This year somehow we are receiving commitments from donors a little bit late, we are talking with donors but we don’t have enough money as of yet," said Inger Marie Vennize, spokeswoman for the UN agency.
"We have had to reduce (the rations) as of this month."
The WFP is talking to the United States - its biggest donor - Germany, Japan and others to secure funds to restore full rations, she added.
"The 50 percent cuts in monthly rations affect over 1.4 million people across Iraq," Vennize said, Reuters reported.
The impact is already being felt in camps east of Mosul, the northern city controlled in part by ISIL. About 160,000 people have been displaced since the military campaign to recapture Mosul from the Daesh was launched in October.
"They gave us a good amount of food in the beginning, but now they have reduced it," said Omar Shukri Mahmoud at the Hassan Sham camp.
"They are giving an entire family the food supply of one person ... And there is no work at all ... we want to go back home," he added.
"We are a big family and this ration is not going to be enough," said 39-year-old Safa Shaker, who has a family of 11.
"We escaped from Daesh in order to have a chance to live and now we came here and they have cut the aid. How are we supposed to live?" she said as she cooked for the family.
About 3 million people have been displaced from their homes in Iraq since 2014, when ISIL took over large areas of the country and of neighboring Syria.