Rebel Houthis in Yemen have denied accusations they were stealing aid "from the mouths of hungry people" levelled by the United Nations food agency, which has threatened to suspend deliveries.

The World Food Programme (WFP), which provides food to millions of malnourished Yemeni people, on Monday accused Houthi rebels of "criminal behaviour" and of selling on much-needed aid on the black market.

David Beasley, WFP director, said that a survey carried by the agency showed that aid is only reaching 40 percent of eligible beneficiaries in the rebel-held capital, Sanaa. Only a third are receiving aid in the rebels' northern stronghold of Saada.

"If you don't act within 10 days, WFP will have no choice but to suspend the assistance ... that goes to nearly the million people," he said, in an unprecedentedly strong warning. "This criminal behavior must stop immediately."

Mohammed al-Houthi, who heads the rebels' Higher Revolutionary Committee, said they had refused to allow the food into the country because it was “rotten” and “violates standards and regulations and is not suitable for human consumption."