Concerns are growing for survivors of flooding in north-west Pakistan after the heaviest rains in a generation killed at least 1,100 people.
There are reports of diarrhoea and cholera among the hundreds of thousands left homeless, and food and drinking water are in short supply.
Pakistan's military is helping with the rescue effort, and the US has announced $10m (£6.4m) in emergency aid.
But some survivors say the government response has been slow and inadequate.
Floodwaters are receding in some areas, but more rain is now forecast.
Part of the main north-south motorway into the region was reopened on Sunday, before reportedly closing again. The brief opening allowed some aid supplies into the flooded area while also permitting people to flee.
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, says the BBC's Aleem Maqbool in the capital.
The information minister of Khyber-Pakhtoonkhwa (formerly North West Frontier Province) - one of the worst-hit provinces - said 1.5 million people had been affected by the floods and landslides.
"We are receiving information about the loss of life and property caused by the floods all over the province," Mian Iftikhar Hussain told the AFP news agency.
The province's disaster management authority earlier said an aerial survey showed dozens of villages had been simply washed away.
Mr Hussain said rescue teams were trying to reach the 27,000 people stranded by the floods in the province, including 1,500 tourists in the Swat district, 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 government's response to the disaster drew a protest of several hundred people in the north-western city of Peshawar, where homeless survivors crammed into temporary shelters overnight.
"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 pleading for clean drinking water to be delivered.
"These people were saying: 'We don't need food at this time but we need drinking water.' All the wells which are providing water for them are full of mud and you cannot use those wells," he told the BBC.
"Already among the children the diarrhoea has been started already and cholera. That's the main risk at this time. Food shortages are already there."
The humanitarian director of Oxfam, Jane Cocking, said the extent of this crisis was only slowly emerging.
"The more villages that are reached the grimmer the picture becomes," she said as the organisation launched an appeal for aid.
"There is a desperate need for temporary shelter, clean drinking water and toilets to avert a public health catastrophe. People also need medical care and basic food items."
The UN's Secretary-General, Ban Ki-moon, said he was "deeply saddened by the significant loss of lives, livelihoods and infrastructure in Pakistan", stressed that the UN would help meet the humanitarian needs of those affected and offered an extra $10m in aid for the relief effort.
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 and time right now is crucial for the survival of so many, our correspondent says.
Earlier, the US promised the government $10m in aid. It also provided about 50,000 meals, four rescue boats and two water-filtration units.
The US embassy in Islamabad said Washington would also be providing 12 temporary bridges to replace some of those destroyed by the floods.
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.
