Speaking in Dubai, the UN agency's Executive Director James Morris said that due to combination of factors, aid workers were struggling to bring relief to the region.
"The aid agencies have managed to give some help to hundreds of thousands of people, but there are an estimated half-a-million more in desperate need, who no-one has managed to reach," he said.
The UN says at least 40,000 people were killed after the earthquake, measuring 7.6 on the Richter scale, struck in Pakistan and Kashmir on 8 October.
Temperatures have been falling to below freezing as winter approaches in the region and thousands of people are at risk of hypothermia as they have only plastic sheeting for shelter.
Large numbers of injured people have been coming down from the Hymalayas on foot and many are suffering from gangrene, according to the WFP.
"People don't just need food, first of all they need shelter, blankets and medical assistance, then food and clean water," Mr Morris said.
Last week the WFP began using helicopters to carry food to villages cut off by road, but bad weather over the weekend grounded these flights and the agency was forced to find alternatives.
"People don't just need food, first of all they need shelter, blankets and medical assistance, then food and clean water"
Executive Director, UN
Now they have begun using packhorses and mules to carry food aid along precipitous paths to reach isolated mountain communities.
Mr Morris said this was one of the toughest challenges they had ever faced: "At the best of times this is very difficult terrain, but with landslides blocking roads, plus the onset of winter, getting to these people is taking far longer, and there is very little time left."