The U.N. food agency is threatening to suspend aid to areas under the control of Yemen's Houthi rebels, if they do not take measures against theft and fraud in food distribution.

The World Food Program said Monday that its "greatest challenge" in Yemen came from "the obstructive and uncooperative role of some of the Houthi leaders in areas under their control."

It says humanitarian workers are being denied access to the hungry, aid convoys have been blocked, and local authorities have interfered with food distribution.

It added that some rebel leaders have repeatedly obstructed the agency's ability to choose how to distribute the food aid.

The Iran-backed Houthis have been at war with a Saudi-led coalition, which backs the internationally recognized government, since 2015.