Torrential monsoon rains in recent weeks have triggered massive floods affecting a fifth of the country.
The catastrophe has affected more than 17m people and has left eight million dependent on aid to survive.
Villagers in the south are fleeing the area where the Indus delta merges towards the Arabian Sea, trailing north in vans laden with furniture or crowded into buses, or in carts pulled by oxen.
Water also lines the road from Hyderabad to Thatta town. Workers are using bulldozers to dig embankments only just higher than the flooding.
The Pakistani government has confirmed 1,600 people dead and 2,366 injured, but officials warn that millions are at risk from diseases and food shortages.
In the southern province of Sindh, where the floods have washed away huge swathes of the rich farmland on which Pakistan's struggling economy depends, a senior administration official warned that fresh floods threaten three towns.
A senior official in Thatta district said 'we have warned people of Sujawal, Mirpur Bathoro and Daro towns to leave for safer places in view of possible flooding there.'
Sujawal, Mirpur Bathoro and Daro towns have an approximate combined population of 400,000.
The Sindh irrigation minister said waters were also mounting pressure on a protective embankment in Garhi Khuda Bakhsh village, where former leaders Benazir Bhutto and her father, as well as her two younger brothers, are buried.
On the Arabian Sea, authorities fear that coastal districts may flood in coming days, trapped by Indus river floods pushing south and rough seas.
The United Nations warned that 800,000 people in desperate need of aid had been cut off by the deluge across the country and appealed for more helicopters to deliver supplies to those people reachable only by air.
In Washington, which has put Pakistan on the front line of efforts to beat back the Taliban in Afghanistan, a US official said Pakistani Taliban were planning to attack foreign aid workers engaged in the relief effort.
On the ground, the International Organization for Migration (IOM) says 4.5m people remain in urgent need of shelter.
Officials warned yet more Pakistanis could be affected in the fertile southern plains of Sindh province, which face the risk of further flooding in the next few days as the major Indus River threatens to burst its banks.
