Torrential rain and flooding have submerged a third of Pakistan and killed more than 1100 people, including 380 children as the United Nations appealed for aid for what it described as an "unprecedented climate catastrophe".
Army helicopters plucked stranded families and dropped food packages to inaccessible areas as the historic deluge, triggered by unusually heavy monsoon rain, destroyed homes, businesses, infrastructure and crops, affecting 33 million people, 15 per cent of the 220 million-strong South Asian nation.
The country has received nearly 190 per cent more rain than the 30-year average in the quarter through August this year, totalling 390.7 millimetres. Sindh province, with a population of 50 million, was hardest hit, getting 466 per cent more rain than the 30-year average.
"One-third of the country is literally under water," Climate Change Minister Sherry Rehman told Reuters, describing the scale of the disaster as "a catastrophe of unknown precedent".
She said the water was not going to recede anytime soon.
At least 380 children were among the dead, Prime Minister Shehbaz Sharif told reporters during a briefing at his office in Islamabad.
"Pakistan is awash in suffering," UN Secretary-General Antonio Guterres said in a video message, as the United Nations launched an appeal for $US160 million ($A232 million) to help the South Asian nation.
"The Pakistani people are facing a monsoon on steroids - the relentless impact of epochal levels of rain and flooding."
Guterres would head to Pakistan next week to see the effects of the "unprecedented climate catastrophe", a UN spokesperson said.
He said the scale of the climate disaster commanded the world's collective attention.
Nearly 300 stranded people, including some tourists, were airlifted in northern Pakistan on Tuesday, a state-run disaster management agency said in a statement, while more than 50,000 people were moved to two government shelters in the northwest.
Pakistan army chief General Qamar Javed Bajwa visited the northern valley of Swat and reviewed rescue and relief operations, saying that "rehabilitation will take a long, long time."
The United States will provide $US30 million in support for Pakistan's flood response through USAID, its embassy in Islamabad said.
Early estimates put the damage from the floods at more than $US10 billion, the government said, adding the world had an obligation to help Pakistan cope with the effects of man-made climate change.
Torrential rain has triggered flash floods that have crashed down from northern mountains, destroying buildings and bridges, and washing away roads and standing and stored crops.
Colossal volumes of water are pouring into the Indus river, which flows down the middle of the country from its northern peaks to southern plains, bringing flooding along its length.
Pakistani Foreign Minister Bilawal Bhutto-Zardari said hundreds of thousands of people were living outdoors without access to food, clean water, shelter or basic healthcare.
Guterres said the $US160 million he hoped to raise with the appeal would provide 5.2 million people with food, water, sanitation, emergency education and health support.