The International Committee of the Red Cross has said that up to 2.5m people across Pakistan have been affected by heavy flooding brought on by torrential monsoon rains.
The ICRC said: 'According to official sources, flooding caused by torrential monsoon rains has killed more than 1,100 people in Pakistan and affected up to 2.5m people across the country in the past week.
'In the worst-affected areas, entire villages were washed away without warning by walls of flood water.'
It also said that thousands of people 'have lost everything'.
Water-borne disease is emerging as a threat to survivors, meaning the death toll could rise even further.
'The floods have killed more than 1,100 people in different parts of Khyber Pakhtunkhwa,' said Mian Iftikhar Hussain, information minister for the province.
'We are receiving information about the loss of life and property caused by the floods all over the province,' he said.
A senior official at the provincial Disaster Management Authority (PDMA) confirmed the toll.
Mr Hussain said more than 3,700 homes have been swept away and the number of people made homeless is mounting.
Hundreds of survivors have sought shelter in schools in Peshawar and Muzaffarabad after escaping the floods.
The US government has announced an initial $10m (€7.6m) aid pledge and has sent helicopters and boats to Pakistan. China, which has also been hit by severe flooding, announced a €1.14m donation.
Mr Hussain said rescue teams are trying to reach 1,500 tourists stranded in Swat district, the scene of a major anti-Taliban military offensive last year.
'We are also getting confirmation of reports about an outbreak of cholera in some areas of Swat,' he said.
The Pakistan Air Force (PAF) said it has airlifted more than 500 stranded people as part of relief operations and is carrying out reconnaissance missions to assess the damage to infrastructure.
Floods also hit parts of Afghanistan, killing at least 65 people and affecting more than 1,000 families.
Pakistani television footage and photographs taken from helicopters showed people clinging to the walls and rooftops of damaged houses as water rushed through villages.
Pakistan's weather bureau said the northwest has been hit by an 'unprecedented' 312mm of rain in 36 hours.
Aid agencies say the true extent of the crisis will not be known until rescue teams gain access to all of the areas affected.
There are also fears that further rain that is forecast for the next 24 hours will make the situation worse.
Aid agencies appeal for help
Hannah Reichardt of Save the Children said aid is not getting through to many areas because the main access roads have been destroyed.
Iftikhar Khalid of Oxfam said: 'This flood is of an overwhelming size and its impact is only slowly emerging.
'The water has swept away everything. Families are desperate about the loss of their loved ones, their belongings and their livelihoods. Water sources and crops are destroyed.
'People are in need of food, clean drinking water, shelter and toilets to avoid a public health crisis.'
Trócaire has appealed for donations from the public.
Emergency manager Morris McQuillan said their immediate priority is to save lives and they are working to get food and water to those hit by the floods.
They are also sending water purification tablets to ensure that water can be made safe to drink.
Trócaire's Pakistan appeal contact number is 1850 408 408.
Concern Regional Director Brid Kennedy said: 'This is a terrible catastrophe affecting some of the most vulnerable people in the world who are living in dire poverty.'
Concern's donations contact number is 1850 410 510.