Climate Change Minister Sherry Rehman said a third of the country was under water, creating a "crisis of unimaginable proportions".
Officials say 1,061 people have died since June when the seasonal rains began, but the final toll could be higher as hundreds of villages in the mountainous north have been cut off after flood-swollen rivers washed away roads and bridges.
The annual monsoon is essential for irrigating crops and replenishing lakes and dams across the Indian subcontinent, but it can also bring destruction.
This year's flooding has affected more than 33 million people, one in seven Pakistanis, said the National Disaster Management Authority.
"It's all one big ocean, there's no dry land to pump the water out," Rehman told AFP, adding the economic cost would also be devastating.
This year's floods are comparable to those of 2010, the worst on record, when more than 2,000 people died.
Flood victims have taken refuge in makeshift camps that have sprung up across the country, where desperation is setting in.
"Living here is miserable. Our self-respect is at stake," said Fazal e Malik, sheltering in the grounds of a school now home to around 2,500 people in the town of Nowshera in Khyber Pakhtunkhwa province.
"I stink but there is no place to take a shower. There are no fans."
Near Sukkur, a city in southern Sindh province and home to an ageing colonial-era barrage on the Indus River that is vital to preventing further catastrophe, one farmer lamented the devastation wrought on his rice fields.
Millions of acres of rich farmland have been flooded by weeks of non-stop rain, but now the Indus is threatening to burst its banks as torrents of water course downstream from tributaries in the north.
"Our crop spanned over 5,000 acres on which the best quality rice was sown and is eaten by you and us," Khalil Ahmed, 70, told AFP.
"All that is finished."
Much of Sindh is now an endless landscape of water, hampering a massive military-led relief operation.
"There are no landing strips or approaches available... our pilots find it difficult to land," one senior officer told AFP.
The army's helicopters were also struggling to pluck people to safety in the north, where soaring mountains and deep valleys make for treacherous flying conditions.
Many rivers in Khyber Pakhtunkhwa province, which boasts some of Pakistan's best tourist spots, have burst their banks, demolishing scores of buildings including a 150-room hotel that crumbled into a raging torrent.
The government has declared an emergency and appealed for international help, and on Sunday the first aid flights began arriving, from Turkey and the UAE.
It could not have come at a worse time for Pakistan, where the economy is in free fall.
In Washington later Monday, the International Monetary Fund executive board was scheduled to meet to decide whether to green-light the resumption of a $6 billion loan programme essential for the country to service its foreign debt, but it is already clear it will take more to repair and rebuild after this monsoon.
Prices of basic goods, particularly onions, tomatoes and chickpeas, are soaring as vendors bemoan a lack of supplies from the flooded breadbasket provinces of Sindh and Punjab.
The met office said the country as a whole had received twice the usual monsoon rainfall, but Balochistan and Sindh had more than four times the average of the last three decades.
Padidan, a small town in Sindh, was drenched by more than 1.2 metres (47 inches) of rain since June, making it the wettest place in the country.
Across Sindh, thousands of displaced people are camped alongside elevated highways and railway tracks, often the only dry spots as far as the eye can see.
More are arriving daily at Sukkur's city ring road, belongings piled on boats and tractor trollies, looking for shelter until the floodwaters recede.
Sukkur Barrage supervisor Aziz Soomro told AFP the main headway of water was expected to arrive around September 5, but was confident the 90-year-old sluice gates would cope.
The barrage diverts water from the Indus into 10,000 km of canals that make up one of the world's biggest irrigation schemes, but the farms it supplies are now mostly under water.
The only bright spark was the latest weather report, with the met office saying there was little chance of rain for the rest of the week.