Hundreds of thousands have been forced from their homes in a disaster blamed on climate change and estimated to have caused losses of about $US10 billion ($A15 billion), disrupting the lives of nearly 33 million of a population of 220 million.

In the southern province of Sindh, one of the worst-hit regions, people worked to strengthen an existing dyke as floodwaters threatened the town of Bhan Syedabad.

Traffic choked the narrow road out of the town as residents fled to nearby villages after evacuating their houses.

"Helicopters are available to rescue the rich, but I had to hire a boat for 2000 rupees to rescue my family," 25-year-old Abdul Fahim, whose nearby home flooded overnight, told Reuters, referring to a sum equivalent to $US9 ($A13).

A short distance away, Pakistan's largest freshwater lake of Manchar is dangerously close to bursting its banks, as high water levels persist despite breaches by authorities to stave off further floods in an effort that displaced 100,000 people.

National disaster authorities said 12 deaths in the last 24 hours carried the toll to 1355. Seven were children, who make up 481 of the dead.

The floods were brought by record monsoon rains and glacier melt in northern mountains. In July and August, Pakistan received 391mm of rain, or nearly double its 30-year average, while Sindh recorded 466 per cent more than the average.

The World Health Organization has said more than 6.4 million people need humanitarian support in the flooded areas.

People in many parts of the country are still struggling with basic necessities weeks after the crisis began.

In the north-western Khyber Pakhtunkhwa province, locals said conditions were poor, with government aid sporadic.

"The water wells are filled with floodwaters and are no longer drinkable," said 62-year-old Moeenullah Khan, from Sar-e-Daryab, a flood-affected town.

"The water standing in the streets and fields is allowing mosquitoes to breed, and almost every second person is suffering from dengue fever, throat, eye and skin infections or diarrhoea."

Dr Nek Dad Afridi, a top local health official, said the provincial government is aware of the health issues facing people and is working with its partners to provide the best available services.

The United Nations has called for $US160 million ($A237 million) in aid. UN Secretary-General Antonio Guterres will tour affected areas and meet senior officials on Friday as part of a two-day visit.

"The secretary-general's visit will further raise global awareness about the massive scale of this calamity and the resulting loss of life and widespread devastation," Pakistan's foreign ministry said in a statement.

Australian Associated Press