Dec. 31 (UPI) -- Houthi rebels are taking humanitarian food aid meant for starving Yemenis and selling it on the black market, the United Nations said Monday.

The organization's World Food Programme said it uncovered the scheme in the rebel-held capital of Sanaa in association with at least one local partner association tasked with handling and distributing food assistance to civilians. The organization was affiliated with the Houthi's Ministry of Education.

"This conduct amounts to the stealing of food from the mouths of hungry people," WFP Executive Director David Beasley said. "At a time when children are dying in Yemen because they haven't enough food to eat, that is an outrage. This criminal behavior must stop immediately."

Earlier this month, the United Nations said the three-year civil war in Yemen has fueled the world's worst humanitarian crisis. Nearly 16 million people in the country have food insecurity ranging from crisis to catastrophe levels, including 10.8 million at crisis levels, 5 million at emergency levels, and 65,000 people at catastrophe levels

The WFP said hungry people are being denied food rations, while others are receiving only partial rations. The organization said it had photographic evidence of fraud showing that records were being falsified and some people are getting aid to which they are not entitled.

Beasley said he's asking Houthis to end the diversion of food aid to ensure it reaches those in need.

"Unless this happens, we'll have no option but to cease working with those who've been conspiring to deprive large numbers of vulnerable people of the food on which they depend. Meanwhile, we're continuing our investigations and addressing those shortcomings which have given rise to this misuse of aid," he said.

Yemen's civil war started in March 2015 as a conflict between Houthi rebels -- made up largely of the Zaidi sect of shiite Muslims -- and the internationally recognized government of President Abdu Rabbo Mansour Hadi, who was run out of the capital and now operates out of the port city of Aden. Saudi Arabia leads a coalition of countries -- including the United States -- in support of Hadi, while Iran backs the rebels.

The Houthis originally supported former Yemeni President Ali Abdullah Saleh, but infighting in December 2017 led to his death.