Swollen rivers are carrying a huge volume of water south, raising fears that further destruction lies ahead. One million people are in the process of evacuating from Sindh province.
Al Jazeera's Imran Khan, reporting from the southern coastal city Karachi, said "we are seeing a number of preparations being made across Sindh province".
"So far 500,000 people have been evacuated. Nearly 250,000 homes have been destroyed across the provinces bordering Sindh.
"The floods are coming further south. Nobody knows whether the floodwaters will reach Karachi, but severe flood warnings have been issued."
Mohammad Bakhsh, a resident of Qasim Ghot village in Sindh province, said at least 20 children from his family have been stranded in the village, holding on to tree branches for survival.
"We are begging the authorities to rescue them," Bakhsh said.
"Two of my children have drowned and we don't know where they are," he said, adding that his cattle died and the cotton crop was destroyed.
Authorities worry that Pakistan, a nation heavily dependent on agriculture, could face a food crisis and economic calamity as crops and infrastructure have been swept away. In many areas, drinking water wells are also full of mud.
In Pakistan's northwest and the Punjab, 12 million people have been displaced, Amal Masud, an official with the National Disaster Management Authority said.
"Things are getting worse. It's raining again. That's hampering our relief work," Masud said.
The Pakistani government has come under fire over its response to the crisis and the prime minister has called for unity to help the flood victims.
"The next two days are very critical in this regard," Yousuf Raza Gilani said.
||13 million affected
||5,500 sq km of farmland under water
"Our top priority is to rescue people, to save their lives. But we will also provide them all facilities, and we will work for their rehabilitation."
Omar Waraich, Pakistan correspondent for the Independent newspaper in the UK, told Al Jazeera: "At this moment, a civilian government is going through hard-times."
But he said that many of the problems were down to years of lack of investment under previous military governments.
"When the military had power infrastructure was not built ... [the] greatest failures [have happened under] military rule.
"Military dictatorships have run this country for half of its history."
Washington has pledged $35m of aid to Pakistan since flooding began.
Australia doubled its aid pledge on Saturday to $4.6m. China and other countries have also contributed significantly.