The number of people known to have been killed by floods in north-west Pakistan has passed 1,100, officials say.
About 30,000 troops have joined the relief effort, with large parts of the north-west submerged by the worst monsoon rains in memory.
There are also fears that with more rain forecast for the next 24 hours, some areas face further threats.
Part of the main north-south motorway into the region was reopened on Sunday, before reportedly closing again.
The opening allowed some aid supplies into the flooded area while permitting some people to flee the region.
The BBC's Aleem Maqbool, in the capital Islamabad, says officials fear that once access to affected areas improves, the full picture will show that the situation is much worse than known so far.
A spokesman for the disaster management authority of Khyber-Pakhtoonkhwa province - formerly known as North West Frontier Province - said an aerial survey was being conducted to determine the full extent of the flooding.
"It has shown that whole villages have been washed away, animals have drowned and grain storages have been washed away," said the spokesman, Latifur Rehman.
"The destruction is massive."
The Pakistani government says 19,000 people in the worst-hit areas had been rescued by soldiers by Saturday night, but that thousands more remained stranded.
About one million people in the north-west of the country are estimated to be affected by the flooding.
Among the hardest-hit areas are:
- The districts of Shangla and Swat, where at least 400 people have been killed and many bridges washed out
- Peshawar, Nowshera and Charsadda, where at least 700 people have drowned
- The Neelam Valley, in Pakistani-administered Kashmir, which has been cut off by landslides.
There have been reports that the flood water is receding in some areas but officials fear that relief operations could be hampered by more rain, with a new monsoon system forecast to arrive in the next 24 hours.
Officials are concerned that more heavy rains could push the flooding south into Sindh province.
Military and rescue workers have been using helicopters to deliver essential supplies to areas that have had transport and communication links cut off.
"Virtually no bridge has been left in Swat. All major and minor bridges have gone, destroyed completely," said army spokesman Maj Gen Athar Abbas.
The army has deployed 43 helicopters and over 100 boats to try to reach people still trapped by the floods, said Mr Rehman.
The Chairman of Pakistan's National Disaster Management Authority, Gen Nadim Ahmed, said it would be necessary to rely on helicopters to shift people and drop aid supplies for some time.
He said the UN was responding to a request for help with food, shelter, water and sanitation and medicines.
The US has also provided about 50,000 meals, four rescue boats and two water-filtration units, said US and Pakistani officials.
The American embassy in Islamabad said it would be providing 12 temporary bridges to replace some of those knocked out by the flooding.
There have been complaints from some that emergency shelters have been inadequate or even non-existent in some areas.
Relief agencies have also warned that there is a risk of disease in the flood-affected areas.
"There is now a real danger of the spread of waterborne diseases like diarrhea, asthma, skin allergies and perhaps cholera in these areas," said Shaharyar Bangash, the head of operations in Khyber-Pakhtoonkhwa for the aid charity World Vision.
As well as the more 1,000 deaths in Pakistan, at least 60 people have died across the border in Afghanistan, where floods have affected four provinces.