Viti Muntarbhorn, the UN human rights investigator for North Korea, said on Monday that the number of people getting UN food aid had dropped to 13,000 from 6.5 million a year ago.
Much of this decline could be attributed to Pyonyang's restrictions on access.
North Korea tested seven missiles in July and then announced an underground nuclear test on October 9, which Muntarbhorn described as a "serious waste of resources".
The UN Security Council responded by imposing sanctions but exempted humanitarian assistance.
"The missile tests had a negative impact on the food situation of the country, since they caused various contributors of humanitarian aid to discontinue providing that aid, while the nuclear test caused further insecurity in the region and beyond," said Muntarbhorn.
North Korea has still not recovered from a brutal famine in the 1990s that experts believe killed about 2.5 million people, or 10 per cent of the population.
Deadly floods earlier this year have also worsened food shortages.
The UN World Food Programme (WFP) also said last month that it had received only eight per cent of the $102 million it needs to provide 150,000 tonnes of food to North Koreans for the next two years.
The WFP said its efforts were already being hampered by a decision by Pyongyang to change its emergency relief programme to a development scheme.
Muntarbhorn appealed to food donors to resume their aid on the basis of "no access, no food", meaning that the aid would be conditional on guaranteed access to North Korean target groups such as women and children and full accountability.