"There is a three-week window of opportunity to deliver assistance to mountainous areas before the first snowfall," the report said. "According to reports from the authorities, severe weather, with heavy rain, is forecast to hit the area in the next three to four days," it added.
The relief effort for quake victims relies heavily on helicopters to reach isolated communities in the region.
Unicef relief spokesman Michael Bociurkiw said children were likely to suffer most if help did not arrive before winter.
"It's going to be very difficult for them to survive," he told AFP here in the devastated capital of Pakistan-administered Kashmir.
The UN estimates that 74,500 people were injured and some 800,000 were without shelter in Azad Kashmir.
The UN warned that time is running out for survivors of the worst catastrophe in the country's history.
Officials say between 10 and 20 percent of affected areas have not received supplies or medical care, despite up to 100 relief flights daily by Pak Army, US, German and Japanese helicopters.
Meanwhile, efforts to reach stranded villagers in northern mountains gathered pace on Monday after the country's friends and foes both urged help for up to 3 million survivors rattled by fresh aftershocks.
Only when the roads are rebuilt - and in some cases this could take weeks - can aid be delivered in sufficient quantities to an estimated 2,000 still inaccessible villages to allow hundreds of thousands of people to survive the rapidly approaching winter.
The fleet of aid helicopters, although growing, cannot reach them all, or deliver sufficient supplies to the worst-hit areas of Azad Kashmir and adjacent North West Frontier Province.
In Rajkot, a two-day trek from the destroyed Muzaffarabad but just a 10-minute helicopter ride, Farid Ayub complained it took 13 days for any aid at all to reach his village.
"There's still a lot of people that are out there, flashing mirrors and waving red flags," said 27-year-old Captain Brandon McRay, a US army helicopter pilot from Texas.
Aid officials say the world is being slow in coming up with the cash to help up to 3 million people who must be sheltered and fed through the winter.
But, with the first heavy snows just five or six weeks away, aid is flowing in faster, said chief United Nations aid co-ordinator Rashid Khalikov, two days before rich countries were to meet in Geneva to discuss help for Pakistan.
"The amounts are increasing - tents, food, non-food items," he said.
Much more is still needed, and Khalikov said the UN was talking to Nato about what it might provide - "everything from excavators to gloves" - in addition to an engineering battalion and medical teams already promised.
Time is short, with night temperatures already below freezing in the hills of Azad Kashmir.
The survivors' most urgent need is shelter. An aid official said 540,000 tents were needed but with global supplies limited, there could be a shortfall of up to 200,000.