The UN cited issues with Houthi leaders who had reportedly been obstructive and uncooperative in areas under their control.
The UN's World Food Program has warned that aid could be suspended to areas of Yemen under the control of Huthi rebels unless they abide by accords aimed at ensuring that supplies are not diverted.
"If the beneficiary targeting and biometric exercise is not carried out as agreed, WFP will be left with no option but to suspend food distributions in the areas controlled by Ansarullah," David Beasley, the agency's executive director, wrote in a letter sent to the rebel movement.
He was referring to accords the WFP signed with the warring parties in December and January.
Herve Verhoosel, spokesman for the WFP's Geneva office, said Monday that the letter was hand-delivered to the Huthi leadership in Yemen.
It was "the second letter WFP has sent requesting greater access", he said.
"WFP noted some progress after the first letter was sent in December, but in recent weeks that progress has halted and in some cases it has been reversed."
The Rome-headquartered WFP lamented that in Yemen "our greatest challenge does not come from the guns... it is the obstructive and uncooperative role of some of the Huthi leaders in areas under their control."
"Humanitarian workers in Yemen are being denied access to the hungry, aid convoys have been blocked, and local authorities have interfered with food distribution," the WFP said in a statement.
"Most importantly, there have been repeated obstacles placed in the way of our independent selection of beneficiaries and a request for a rollout of a biometric registration system."
"This has to stop," WFP said.
The Yemen conflict has killed tens of thousands of people since a Saudi-led military coalition intervened in support of the beleaguered government in March 2015, according to the World Health Organization.
The fighting has triggered what the United Nations describes as the world's worst humanitarian crisis, with 3.3 million people still displaced and 24.1 million - more than two-thirds of the population - in need of aid.