The worst monsoon rains in 80 years are continuing to sweep from the north-west to south and central Pakistan.
Rivers in Sindh province, home to Karachi, Pakistan's biggest city and business hub, are bursting their banks.
Pakistani authorities have evacuated 500,000 people in 11 districts of Sindh and issued warnings to people in low-lying areas of the Indus river.
Flooding has submerged whole villages in the past week, killing about 1,600 people and affecting another 4.5m.
There is mounting anger at the absence of President Asif Ali Zardari, who left the country for a state visit to Britain to meet the UK Prime Minister David Cameron.
With flood victims bitterly accusing the authorities of failing to come to their aid, the disaster has piled yet more pressure on an administration struggling to contain Taliban violence and an economic crisis.
And the region is only midway through monsoon season, with more rain forecast.
A big wave of flood waters has been passing through Guddu barrage in the upper parts of Sindh and is heading down to Sukkur barrage, the BBC's M Ilyas Khan in Islamabad says.
Electricity grid stations in the Punjab-Sindh region have been shut down to minimise chances of electrocution.
Officials say they are trying to move about one million people from the riverine region of Sindh.
So far, 263,000 houses have been fully or partially damaged in the two worst affected provinces of Khyber Pakhtunkhwa and Punjab.
According to the Federal Flood Commission, 1.4m acres (557,000 hectares) of crop land has been flooded across the country and more than 10,000 cows have perished.
Waters in the Punjab region are now receding but several new villages in the Jaffarabad area have been deluged since Thursday evening.
There have been fresh downpours in the north-west, adding to the misery of over two million people left homeless there by the floods.
About 1.4 million people have been displaced in Punjab, according to a UN spokesman.
The situation is likely to worsen as the meteorological department has predicted heavy rains in areas already hit by the floods.
"We're forecasting widespread rains in the country, specially in flood-affected areas," Reuters quoted Qamar-uz-Zaman Chaudhry, director general of the department, as saying.
Authorities in Sindh have warned of major floods in the next 48 hours along the swollen Indus river.
"It is unprecedented floods in our history," news agency AFP quoted military spokesman Major General Athar Abbas as saying. "We do not have the kind of resources to cope with a situation like this."
Army helicopters have evacuated about 3,000 tourists stranded in the Kalam region of Swat district, which has been cut off after many bridges were washed away by flood waters.
Sixty boats have been sent from Lahore to Multan, in Punjab province, and Sukkur in Sindh province, for evacuation purposes, an army press release said.
Officials say the most immediate needs of the displaced are tents, plastic sheets, food and medicines.
The World Bank said it had set up an aid fund at Pakistan's request with initial pledges of $80m (£50m).
Meanwhile, 90 people have died in flash floods in the Ladakh region of Indian-administered Kashmir, officials say; nearly 270 people were injured.
The inspector general of Kashmir police Farooq Ahmed told the BBC that the dead included four policemen who had joined the rescue operation.