The United Nations food agency demanded on Friday that the Houthi rebels put an “immediate end” to food diversion and hold accountable those responsible for food theft, but said it not clear how much of their aid is actually reaching Yemenis.

Meanwhile, the Saudi Ambassador to Yemen, Mohammed bin Saeed Al-Jaber, told The Associated Press that at least 60 percent of the funds sent to the U.N. by members of the Saudi-led coalition battling the Houthis have not yet been spent on their intended purpose. Obstacles imposed by the Houthis on aid agencies in northern Yemen, such as blocking access to on-the-ground medical programs, have resulted in the funds remaining held up, he alleged.

The latest remarks come shortly after an investigation by the AP found that across Yemen, factions and militias on both sides of the conflict have blocked food aid from reaching groups suspected of disloyalty, diverting it instead to front-line combat units or selling it for a profit on the black market.

“This is an issue that affects not just WFP but all aid agencies working in Yemen and indeed in war zones everywhere,” said Herve Verhoosel, spokesman for the World Food Program, on Friday in Geneva. “No-one can say for certain how widespread this problem is,” he added.

“The de facto authorities in Sana’a have a responsibility to take action against those involved in stealing from the beneficiaries and in trading of food aid,” he said, adding that the WFP has repeatedly demanded that the Houthis introduce biometric registration to bring an end to fraud and aid losses.

He added that there will be no major shift from food baskets to cash transfers for beneficiaries — a demand by the Houthis— until there are clear monitoring and verification methods in place, along with biometric registration.

“Given the risk of corruption, we have made it clear to the de facto authorities that we will not introduce cash-based transfers unless we are authorized to implement a biometric identification system that uses personal data including iris scans and ten-finger prints to ensure that only registered beneficiaries are able to claim their cash or food rations,” he said.AP