Heavy rain pounded parts of Pakistan Friday after the government declared an emergency to deal with monsoon flooding that it said had affected more than four million people.

The annual monsoon is essential for irrigating crops and replenishing lakes and dams across the Indian subcontinent, but each year it also brings a wave of destruction.

The National Disaster Management Agency (NDMA) said Friday that more than 900 people had been killed this year -- including 34 in the last 24 hours -- as a result of the monsoon rains that began in June.

Officials say this year's floods are comparable to 2010 -- the worst on record -- when over 2,000 people died and nearly a fifth of the country was under water.

"I have never seen such huge flooding because of rains in my life," octogenarian farmer Rahim Bakhsh Brohi told AFP near Sukkur, in southern Sindh province.

Like thousands of others in rural Pakistan, Brohi was seeking shelter beside the national highway, as the elevated roads are among the few dry places in the endless landscapes of water.

The disaster agency said over 4.2 million people were "affected" by the flooding, with nearly 220,000 homes destroyed and half a million more badly damaged.

Two million acres of cultivated crops had been wiped out in Sindh alone, the provincial disaster agency said, where many farmers live hand-to-mouth, season-to-season.

"My cotton crop that was sown on 50 acres of land is all gone," Nasrullah Mehar told AFP.

"It's a huge loss for me... what can be done?"

Climate Change Minister Sherry Rehman, who on Wednesday called the floods "a catastrophe of epic scale", said the government had declared an emergency, and appealed for international assistance.

Pakistan is eighth on the Global Climate Risk Index, a list compiled by the environmental NGO Germanwatch of countries deemed most vulnerable to extreme weather caused by climate change.

Earlier this year much of the nation was in the grip of a drought and heatwave, with temperatures hitting 51 degrees Celsius (124 Fahrenheit) in Jacobabad, Sindh province.

The city is now grappling with floods that have inundated homes and swept away roads and bridges.

In Sukkur, about 75 kilometres (50 miles) away, residents struggled to make their way along muddy streets clogged with flood-borne debris.

"If you had come earlier the water was this high," 24-year-old student Aqeel Ahmed told AFP, raising his hand to his chest.

Prime Minister Shehbaz Sharif cancelled a planned trip to Britain to oversee the flood response, and ordered the army to throw every resource into relief operations.

"I have seen from the air and the devastation can't be expressed in words," he said on state TV after visiting Sukkur.

"The towns, villages and crops are inundated by the water. I don't think this level of destruction has taken place before."

A national fundraising appeal has been launched, with Pakistan's military saying every commissioned officer would donate a month's salary towards it.

The worst-hit areas are Balochistan and Sindh in the south and west, but almost all of Pakistan has suffered this year.

Images were circulating on social media Friday of swollen rivers obliterating buildings and bridges built along their banks in the mountainous north.

In Chaman, the western frontier town neighbouring Afghanistan, travellers had to wade through waist-high water to cross the border after a nearby dam burst, adding to the deluge brought by rain.

Pakistan Railways said nearby Quetta, capital of Balochistan province, had been cut off and train services suspended after a key bridge was damaged by a flash flood.