Some half a million survivors of the South Asian earthquake have still received no help at all, the UN's World Food Programme (WFP) says.
Medical aid is only reaching a fraction of the victims
The warning comes amid growing fears that many more people could die from winter weather and untreated injuries.
Doctors in Pakistan-administered Kashmir say tens of thousands in the state have still to receive treatment.
More than 40,000 people are confirmed dead in the earthquake. Local officials say the toll may be as high as 54,000.
Many of the survivors are in remote mountains or deep valleys with no medical help available.
'Little time left'
"The aid agencies have managed to give some help to hundreds of thousands of people," the executive director of the WFP, James Morris, said on Tuesday.
"But there are an estimated half a million more people out there in desperate need, who no one has managed to reach."
Mr Morris warned that relief workers were presented with one of the toughest challenges they had ever faced.
"There is very little time left."
The UN says more than three million people have been left homeless and the Pakistani government says it needs another 500,000 tents capable of withstanding winter conditions.
Heavy duty tent designed for long-term use by a single family
PVC groundsheet sewn onto the sides for windproofing and to retain warmth
Some types are designed to accomodate cooking stoves
ICRC estimates that 30,000 such tents are required in Pakistan
One UN official in Pakistan says the deep valleys and high mountains of Kashmir are less accessible for relief workers than the area affected by the 2003 Bam earthquake in Iran or the coastal regions devastated in last year's tsunami.
"Here we've got over 15,000 villages spread out through the affected region," Andrew McLeod, operations manager of the UN Emergency Response Team working out of Islamabad, told the BBC.
"The affected areas are much larger in geographical size than the tsunami, and rather than being in flat coastal areas, we are operating in some of the highest mountains and deepest valleys in the world."
The medical situation in some of the remote areas is being described as tragic by one aid official, with many of the injured facing death unless they receive immediate medical attention.
Sebastian Novak of the International Committee of the Red Cross flew by helicopter to reach the remote village of Chaka, which had not been previously reached.
The UN says many more tents are needed to protect the victims
He told the BBC that 25% of the patients have had their limbs amputated.
"I had a young lady today, she was about 20, both legs amputated at the knee, and her right arm off, so she basically only had her left arm left.
"There are children amputated, old people amputated. Everybody is going straight to amputation," he said.
Improved weather on Tuesday has meant that helicopters have resumed operations in Pakistan-administered Kashmir.
Soldiers used mules to reach victims living in steep villages along some of the region's remote valleys.
But aid agencies and correspondents say the need for more helicopters remains the most urgent priority in Pakistan-administered Kashmir.
On Monday, India and Pakistan failed to agree terms under which Indian army aircraft would join the search-and-rescue mission to reach earthquake survivors.
Pakistan says it will accept Indian helicopters but not crew, while India insists its pilots must fly the craft.
India has suggested it will allow Pakistani aircraft to carry out relief operations on its side of the Line of Control in Kashmir, and offered to work on the Pakistani side.
Islamabad said it was unacceptable for Indian military personnel to be operating in Pakistani-administered Kashmir.
The BBC's Aamer Ahmed Khan in Karachi says an agreement between the historic rivals could potentially double the size of the fleet of relief helicopters operating in Pakistan and Pakistan-administered Kashmir.
Pakistan's government puts the overall number of deaths in Pakistan-administered Kashmir and North West Frontier Province at about 40,000.
In Indian-administered Kashmir, officials say 1,400 people were killed.
* Many roads in the affected area are damaged and/or impassable