Prime Minister Shehbaz Sharif called the flooding "the worst in the history of Pakistan".
The rains that began in June have unleashed powerful floods across the country. (File)
Islamabad: Aid efforts ramped up across flooded Pakistan on Tuesday to help tens of millions of people affected by relentless monsoon rains that have submerged a third of the country and claimed more than 1,100 lives.
Prime Minister Shehbaz Sharif called the flooding "the worst in the history of Pakistan", adding it would cost at least $10 billion to repair damaged infrastructure spread across the country.
The rains that began in June have unleashed powerful floods across the country that have washed away swathes of vital crops and damaged or destroyed more than a million homes.
Authorities and charities are struggling to accelerate aid delivery to more than 33 million people, a challenging task in areas cut off because many roads and bridges have been critically damaged.
Displaced people have been wandering what dry land remains, seeking shelter, food and drinking water.
"For God's sake help us out," said Qadir, 35, who was camped out with his extended family on a road near the southern city of Sukkur.
"We walked along the road for three days to reach here. There's nothing left back at home, we only managed to save our lives."
In the country's south and west, many Pakistanis have crammed onto elevated highways and railroad tracks to escape the flooded plains.
"We don't even have space to cook food. We need help," Rimsha Bibi, a schoolgirl in Dera Ghazi Khan in central Pakistan, told AFP.
Pakistan receives heavy -- often destructive -- rains during its annual monsoon season, which are crucial for agriculture and water supplies.
But such intense downpours have not been seen for three decades.
Pakistani officials have blamed climate change, which is increasing the frequency and intensity of extreme weather around the world.
"To see the devastation on the ground is really mind-boggling," Pakistan's climate change minister Sherry Rehman told AFP.
"When we send in water pumps, they say, 'Where do we pump the water?' It's all one big ocean, there's no dry land to pump the water out."
She said "literally a third" of the country was under water, comparing scenes from the disaster to a dystopian movie.
The Indus River, which runs the length of the South Asian nation, is threatening to burst its banks as torrents of water rush downstream from its tributaries in the north.
Pakistan as a whole has been deluged with twice the usual monsoon rainfall, the meteorological office said, but Balochistan and Sindh provinces have seen more than four times the average of the last three decades.
'Monsoon on steroids'
The disaster could not have come at a worse time for Pakistan, where the economy is in free fall.
Appealing for international help, the government has declared an emergency.
Aid flights have arrived in recent days from Turkey and the United Arab Emirates, while other countries including Canada, Australia and Japan have also pledged assistance.
The United Nations launched a formal $160 million appeal on Tuesday to fund emergency aid.
"Pakistan is awash in suffering. The Pakistani people are facing a monsoon on steroids -- the relentless impact of epochal levels of rain and flooding," UN Secretary-General Antonio Guterres said in a video statement, calling it a "colossal crisis".
PM Sharif promised donors that any funding would be responsibly spent.
"I want to give my solemn pledge and solemn commitment... every penny will be spent in a very transparent fashion. Every penny will reach the needy," he said.
Pakistan was already desperate for international support and the floods have compounded the challenge.
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.
There was some relief on Monday when the International Monetary Fund approved the revival of a loan programme for Pakistan, releasing a $1.1 billion tranche.
Makeshift relief camps have sprung up all over Pakistan -- in schools, on motorways and in military bases.
In the northwestern town of Nowshera, a technical college was turned into a shelter for up to 2,500 flood victims.
They sweltered in the summer heat with sporadic food aid and little access to water.
"I never thought that one day we will have to live like this," said 60-year-old Malang Jan.
"We have lost our heaven and are now forced to live a miserable life."