The worst flooding in Pakistan's history has now affected more than four million people and left at least 1,600 dead, says the UN.
While floods in the north-west began to recede, the vast body of water has been moving down the country into new parts of Punjab and menacing Sindh province.
All wells have been contaminated and water-borne diseases are spreading, officials say.
The region is midway through monsoon season and more rain is forecast.
The number of affected districts in Punjab has reached seven, while 350,000 people have been moved from neighbouring Sindh province, most of which is on high alert, the United Nations said.
Manuel Bessler, who heads the UN Office for the Co-ordination of Humanitarian Affairs in Pakistan, told a news conference in Geneva: "What we are facing now is a major catastrophe."
He added: "We are only in the middle of the monsoon season, there is more rain expected. We are afraid it will get worse."
With crops, homes, roads and bridges washed away, the human exodus continued on Thursday as yet more torrential rain fell.
Fleeing villagers have waded barefoot through water up to their necks and chests, carrying belongings on their heads.
In Punjab, known as Pakistan's "breadbasket" for its rich agriculture, more than 1,300 villages have been affected and at least 25,000 homes destroyed, said disaster relief officials in the province.
In the worst-affected areas, small villages have been submerged.
In large tracts of Kot Addu and nearby Layyah, water levels were so high only treetops were visible.
In Pakistani-controlled Kashmir, at least 20 people died when a bus plunged into a rain-swollen river.
At a refugee camp in Charsadda, Khyber Pakhtunkhwa (formerly North West Frontier) province, Tahir Shah, a doctor, said most patients coming to him were suffering from stomach problems, chest infections and skin problems, caused mainly by dirty flood water.
Meanwhile, forecasters warned of further downpours for the north-west; the region has not seen such flooding since 1929.
"The flood water is increasing at different points and we are expecting more rain in the next 24 hours," Hazrat Mir, chief meteorologist for Punjab, told news agency AFP.
UN Secretary General Ban Ki-moon has sent a special envoy, Jean-Maurice Ripert, to Pakistan to help mobilise international support and aid flood victims.
The army has used boats and helicopters to evacuate stranded villagers to higher ground.
Prime Minister Yousuf Raza Gilani has said about 100,000 people have been rescued.
But government and civilian agencies have been struggling to get supplies to the worst affected areas.
Victims have bitterly accused the authorities of failing to come to their rescue and provide sufficient relief.
Particular scorn has been poured on President Asif Ali Zardari because he pressed ahead with a visit to Europe.
Mr Zardari is due to launch his son's political career on Saturday in the British city of Birmingham.
The disaster has piled yet more pressure on an administration struggling to contain Taliban violence and an economic crisis.
Meanwhile, local Islamic charities with unconfirmed links to militant groups have reportedly been stepping into the breach to help flood victims.