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 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. We are only in the middle of the monsoon season, there is more rain expected. We are afraid it will get worse."

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.

The army has used boats and helicopters to evacuate stranded villagers to higher ground.

Prime Minister Yousuf Raza Gilani said about 100,000 people have been rescued.