The International Committee of the Red Cross said up to 2.5 million people across Pakistan have been affected by the flooding.
"In the worst-affected areas, entire villages were washed away without warning by walls of flood water," it said in a statement, noting that thousands of people "have lost everything".
Aid workers and Pakistan's military conducted what relief efforts they could as officials warned that the death toll was rising.
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Death toll rises
"There are 774 deaths registered with us, but the total number killed in the flood is 1200 to 1500," Mian Iftikhar Hussain, the information minister for north-west province Khyber Pakhtunkhwa, told a news conference in Peshawar.
Later, Hussain told AFP the floods had displaced 500,000 people in Khyber Pakhtunkhwa province and put the figure of people affected by the floods at more than 1.5 million.
Syed Zahir Ali Shah, health minister for the province, said about 100,000 people, mostly children, were suffering from illnesses such as gastroenteritis.
A spokesman for the charity World Vision said teams had visited those affected around the main north-western city of Peshawar, but those further north had been inaccessible by road.
"They don't have drinking water or food. They said there have been some visible signs of water-borne diseases," Muhammad Ali told AFP, warning that the death toll was likely to rise further as aid workers reached more areas.
At a camp set up by the army for about 640 families in Nowshehra, women and children ran after vehicles bringing food and water, pushing and shouting.
"We are getting patients with trauma, gastroenteritis, skin diseases and dehydration," doctor Shoaib Mohammad told AFP at a small 20-bed mobile clinic.
Fifty-year-old Ajmair Shah went into shock after flood destroyed his home in Nowshehra. He lay motionless in his hospital bed and staring into the air.
"My house was swept away by the flood, nothing is left there. I have lost everything," Shah said and started weeping.
People at the camp said there were no proper latrines or bathrooms and that the only respite from the crushing heat was plastic hand fans. Most of them fled in the clothes they were wearing and many children roamed around naked.
"They throw food at us as if we are animals and not humans," one man, Ilyas Khan, told AFP, complaining there was no proper system of distribution.
Aid filters in from other nations
UN chief Ban Ki-moon pledged aid of up to $US10 million ($A11 million) for those affected by the crisis, saying he was "deeply saddened" by the floods.
The US government announced a $US10 million aid pledge and has rushed helicopters and boats to Pakistan. China has also promised $US1.5 million ($A1.66 million), according to the official Xinhua news agency.
Britain also pledged STG5 million ($A8.67 million) on Monday to help the victims of flooding.
Anwer Kazmi, a spokesman for Pakistan's largest charity, the Edhi Foundation, said at least 1256 people had been killed.
Khyber Pakhtunkhwa chief minister Amir Haider Hoti said the floods were "unprecedented" and warned it could take up to 10 days to assess the overall number of dead and displaced.
Pakistan's meteorological service forecast rains of up to 200mm in the next weeks across the north-west, Pakistani-administered Kashmir, the central province of Punjab and Sindh in the south.
Flood victims have condemned authorities over a sluggish relief, shouting "give us aid sent by foreign countries" and "death to the corrupt government."
The National Disaster Management Authority (NDMA) said it had rescued more than 28,000 people in Khyber Pakhtunkhwa by helicopter and boat.
The NDMA said nearly 30,000 homes had been damaged across the country.
In addition to those killed in the north-west, local officials said 53 people died in Pakistani Kashmir, 26 in the south-western province of Baluchistan and 49 in Punjab, Pakistan's most populous province.
The southern province of Sindh went on red alert, spokesman Mazhar Siddiqui said, fearing that 150,000 people could be displaced by expected floods there.
