"It is the toughest physical natural disaster the World Food Programme has ever seen," he told a news conference after visiting quake-hit areas in Pakistani Kashmir and neighbouring North West Frontier Province.

The Oct. 8 quake killed more than 73,000 people and made more than 3 million homeless. There are fears that many poorly nourished survivors may not survive the harsh Himalayan winter.

Morris said the WFP had bolstered food rations to provide extra calories needed for people to survive the cold weather.

He said the agency had accepted the responsibility of providing food for 400,000 people who could be reached by air and 600,000 others reachable by land.

However, he said bad weather and landslides would inevitably disrupt the flow of supplies.

Aid officials say most of the tents distributed were incapable of withstanding the snow and rain and the focus of relief efforts was on ways of ensuring people stayed warm.

Most survivors want to remain on their land with their animals but thousands have trekked out of the mountains to towns in the foothills where crowded, unsanitary camps have sprung up.

While there have been no outbreaks of disease or increase in the mortality rate since the winter began a week ago, conditions were conducive to the spread of disease.

Sardar Mehmood Ahmed, district health officer in Muzaffarabad, the devastated capital of Pakistani Kashmir, said a 10-month old boy died from measles on Thursday and at least 14 other cases of the disease were reported in one camp.