"Keeping the people dry, keeping the people warm, well-fed and healthy remains a colossal job," chief UN humanitarian coordinator Jan Vandemoortele told a news conference.

"The situation remains very difficult and actually we are on a knife's edge," he said.

A huge aid effort involving Pakistani authorities, the United Nations, the Red Cross and numerous aid groups, has been trying to ensure survivors get proper shelter and adequate food to survive the winter.

Tents have been delivered to mountain villages by helicopter, truck, on donkeys and on foot in the eight weeks since the quake struck.

"The bad news is that not all tents are providing adequate shelter, given the weather conditions," Vandemoortele said.

"Many can be winterised, many are being winterised but some will have to be replaced altogether," he said.

Corrugated iron sheeting was essential for helping to protect tents, or to help families to build one room that could be heated. Supplies of the sheets had to be maintained, he said.

SHELTER KITS

Darren Boisvert of the International Organisation for Migration which is overseeing efforts to provide shelter, said 90 percent of the 420,000 tents distributed were not suitable for winter.

There were just not enough winter tents in the world to meet the need and people now had to be helped to build their own shelter out of the ruins of their old homes, Boisvert said.

The immediate goal was to get 10,000 winter shelter kits to people living at altitudes above 5,000 feet (1,500 metres). The kits include corrugated iron sheeting and other basic building material.

"The reason for this is quite simple. The more people that we can properly shelter in these upper elevations means that less people will move down ... into camps below," he said.

"Our main focus it to keep people in their homes," he said.

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

"I tried to rebuild my house and give my cattle shelter but I don't have building material and I couldn't get a tent," said one man taking his family down from their mountain home to the low land.

The man, Mohammad Ashraf, encountered on a road in North West Frontier Province, said he wanted to get his family settled in a town then he would head back to look after his cattle.

Speaking earlier, Vandemoortele said towns such as Muzaffarabad, capital of Pakistani Kashmir, could not take in any more people seeking shelter. "If the people come down from the valleys by the hundreds and thousands, we will face a major challenge," he said. "The population of the city is already bigger than it has ever been, it is over full. There is no space for additional camps."

While there had been no outbreaks of epidemics or increase in mortality among survivors since the winter began to bite a week ago, conditions for the spread of disease were ripe.

"Acute respiratory infection, in particular, is the biggest concern we have and that is why we focus so much on shelter."