Up to 2.5 million people have been affected by devastating floods in north-west Pakistan, the International Red Cross has said.
Rescuers are struggling to reach 27,000 people still cut off by the floods, which are the worst in 80 years.
At least 1,100 people have died and thousands have lost everything.
"In the worst-affected areas, entire villages were washed away without warning by walls of flood water," the Red Cross said in a statement.
There are fears diarrhoea and cholera will spread among the homeless. Food is scarce and water supplies have been contaminated by the floods.
Mian Iftikhar Hussain, the Information Minister of Khyber-Pakhtoonkhwa (formerly North West Frontier Province), one of the worst-hit regions, said rescue teams were trying to reach 27,000 stranded people, including 1,500 tourists in the Swat Valley, the scene of a major military offensive against the Taliban last year.
"We are also getting confirmation of reports about an outbreak of cholera in some areas of Swat," he added.
The Pakistani military says it has committed 30,000 troops and dozens of helicopters to the relief effort, but winching individuals to safety is a slow process.
The army - which says it has rescued 28,000 people in recent days - predicts the initial search and rescue operation will take up to 10 days, says the BBC's Orla Guerin, who has been on board a military helicopter over the Swat Valley.
But the army says rebuilding the damaged areas could take six months or more.
A spokesman for the UK-based charity Save the Children told the BBC that the infrastructure damage in Swat may be worse than in the earthquake which devastated the region in 2005.
"We fear that in places that have not been accessed as yet there are people that were trapped, and there is a possibility of more deaths taking place," the spokesman said.
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.
The BBC's Aleem Maqbool in Islamabad says the biggest challenge for the emergency services is access, as so many areas had their transport and communication links destroyed and are now isolated.
Officials in Islamabad fear that once access to affected areas improves, the full picture will show that the situation is much worse than is so far known, our correspondent adds.
Floodwaters receded in some areas as weather conditions improved on Monday, but more rain is now forecast.
Part of the main north-south motorway into the region was re-opened on Sunday, before reportedly closing again. The brief opening allowed some aid supplies into the flooded area while also permitting people to flee.
The rain may have stopped but huge swathes of north-west Pakistan remain submerged, with many of those affected still stranded and waiting for help.
There have been complaints from some survivors that the government response has been slow and inadequate.
Several hundred people took part in a protest in the north-western city of Peshawar, where homeless survivors have crammed into temporary shelters.
"The government is not helping us," said 53-year-old labourer Ejaz Khan, whose house on the city's outskirts was swept away by the floods.
"The school building where I sheltered is packed with people, with no adequate arrangement for food and medicine," he told AFP news agency.
Shariyar Khan Bangash, the regional programme manager for the aid organisation World Vision, based in Peshawar, said survivors of the worst-affected areas were desperate for drinking water.
"All the wells which are providing water for them are full of mud," he told the BBC. "Among the children the diarrhoea has started already, and cholera."
UN Secretary General Ban Ki-moon said he was "deeply saddened by the significant loss of lives, livelihoods and infrastructure in Pakistan", and offered an extra $10m (£6.5m) in aid for the relief effort.
The UK government's Department for International Development has said it is providing £10m for the Pakistan relief effort.
International Development Secretary Andrew Mitchell said: "I know many British people are deeply concerned by the terrible suffering caused by the ongoing monsoon floods in Pakistan. The government of Pakistan is leading the relief efforts and the UK is ready to help in any way we can."
Earlier, the US also promised the government $10m in aid.
The US embassy in Islamabad said Washington would also be providing 12 temporary bridges to replace some of those destroyed by the floods.