Thousands of people fled on Friday from the southern Pakistani town of Thatta after the swollen Indus river burst its banks and authorities ordered an evacuation.
Fresh flooding has sent a million people fleeing from their homes in the south in the past 48 hours, the United Nations said.
The death toll from the floods, triggered by unusually heavy monsoon downpours over the upper Indus basin a month ago, was expected to rise significantly as more bodies were found while many people were missing, a disaster authority spokeswoman said.
Floodwaters are beginning to recede across most of the country as the water flows downstream, but high tides in the Arabian Sea meant they still posed a threat to towns in Sindh province such as Thatta, 70 km (45 miles) east of Karachi.
Concern continues to be the south, U.N. spokeswoman Stacey Winston told a news conference. In the last 48 hours nearly one million people have been displaced.
The U.N. earlier said the floods had forced about six million people from their homes.
A stream of buses, cars, trucks and bullock carts snaked out of Thatta heading for higher ground. Many people were walking, driving livestock and carrying bundles of possessions.
But some people refused to go.
We're not going to leave. How can we leave? Who will protect my house? said fisherman Bali Bhal sitting by the road.
The town has not been flooded but officials said they were taking no chances.
Our biggest apprehension is if we are unable to control the water and road access is cut, then it would be very difficult to mobilise resources and evacuate people, said provincial disaster management director Saleh Farooqi.
Many people from outlying areas had taken refuge in Thatta, which normally has a population of about 300,000, and now had to move again, another officials said.
But Farooqi said one breach in the river bank had been plugged and authorities were working to patch another.
The southern business hub of Karachi, Pakistan's largest city, is far away from the flood zone.
The official death toll is about 1,600 people but a spokeswoman for the National Disaster Management Authority said that number would go up.
Some of them may be found and some of may not, the spokeswoman, Amal Masud, said of the many missing. There can be a significant increase in the death toll but it will not be an alarming figure.
The floods are Pakistan's worst ever natural disaster in terms of the amount of damage and the number of people affected.
The floods have deepened anger with the government of President Asif Ali Zardari, which was already perceived as ineffective and corrupt and there are fears of social unrest.
In the United States, an ally which regards Pakistan as a front-line state in its war against the Taliban, concerns have grown that Islamist charities linked to militants had increased their involvement in the flood relief effort, possibly exploiting anger to gain recruits.[nN26226768]
The government said last week banned militant groups would not be allowed into flooded areas but a spokesman for a charity linked to the Lashkar-e-Taiba militant faction said his group had no political agenda and was still providing relief.
The people of Pakistan trust us and are giving us money, said Yahya Mujahid, a spokesman for the Jamaat-ud-Dawa charity.
Even before the floods, Pakistan's economy was fragile.
Economic growth, forecast at 4.5 percent this fiscal year, is now predicted at anything between zero to 3 percent.
Finance Minister Abdul Hafeez Shaikh, in Washington for talks with the International Monetary Fund (IMF), said Pakistan wanted to keep pursuing an $11 billion IMF loan programme and demonstrate its resolve to make tough decisions, dismissing reports that Pakistan might abandon the programme. [nN26213669]
The floods have damaged at least 3.2 million hectares (7.9 million acres) -- about 14 percent of the entire cultivated land -- according to the United Nation's food agency. The total cost in crop damage is believed to be about $3 billion.
Food Ministry officials said the government was likely to cancel plans to export 2 million tonnes of surplus to ensure there are no shortages. Up to 75,000 tonnes of stored grains have been washed away. [nSGE67P0M0]
(Additional reporting by Zeeshan Haider, Rebecca Conway, Augustine Anthony in Islamabad; Phil Stewart, Sue Pleming in Washington; Writing by Robert Birsel; Editing by Chris Allbritton and Sanjeev Miglani))