Pakistan began evacuating half a million people from flood-risk areas in the south on Thursday as the overall number hit by the country's worst floods in living memory rose to more than four million.

The United Nations rushed a top envoy to Pakistan to mobilise international support and address the urgent plight of millions affected by torrential monsoon rains across the volatile country that have killed around 1,500.

The disaster is now into its second week and the rains are spreading into Pakistan's most populous provinces of Punjab and Sindh, as anger mounts against the government response after villages and farmland were washed away.

In Sindh, authorities warned that major floods were expected on Saturday and Sunday in fertile agricultural areas along the Indus river.

The military said 25,000 people had been evacuated in parts of the province while the local government put the number at 150,000.

"We have a target evacuation of at least 500,000 people who live in 11 most vulnerable districts," said provincial irrigation minister Jam Saifullah Dharejo, saying many towns and villages were in danger.

Maurizio Giuliano, a spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA), said rising water levels could pose threat to Sukkur Barrage.

Further north in Punjab, an AFP reporter saw an exodus of people streaming out of flooded villages, wading barefoot through water, cramming belongings onto donkey carts and into cars under heavy rains.

"Altogether, more than four million people are in one way or another affected," said Manuel Bessler, who heads the OCHA in Pakistan.

"What we are facing now is a major catastrophe," he said.

Officials warned that dams could burst as heavy rain lashed the Punjab town of Kot Addu, transforming the area into a giant lake.

Army helicopters flew overhead as people streamed out of flooded villages searching for safer ground.

"All these villages are dangerous now. We are evacuating the population. Important installations are in danger," said Manzoor Sarwar, police chief for Muzaffargarh district.

Survivors lashed out at authorities for failing to come to their rescue and provide better relief, piling pressure on a cash-strapped administration straining to contain Taliban violence and an economic crisis.

"Everybody is leaving. We came out empty-handed. We didn't have enough time to take our belongings," Ghulam Mustafa, 26, told AFP in Mehmood Kot, a village about six hours' drive south of Lahore, the capital of Punjab.

Houses, shops, petrol pumps and small villages were submerged. Villagers waded barefoot through water up to their necks and chests, carrying belongings on their heads.

"There's up to six feet (1.8 metres) of water in the city. All our stuff was destroyed. We saved only our children," Sadaqat Ali, 28, a plumber from Kot Addu told AFP.

His 12-member family carried bags on their heads, exhaustion and unhappiness etched on their faces. The children were barefoot.

Suhail Tipu, a senior administration official in the area, said that engineers had breached a flooded canal in two places to protect the Kot Addu power station, one of the country's biggest.

UN special envoy Jean-Maurice Ripert visited affected areas in the northwest, where officials say there has been a lull in rainfall and water levels are receding.

Provincial information minister Mian Iftikhar Hussain told reporters that 4.2 million people had been affected while 720,000 houses were destroyed across Khyber Pakhtunkhwa.

In Pakistani-administered Kashmir, a passenger bus plunged into the flooded Jhelum river on Thursday, killing 22 people.

Although Prime Minister Yousuf Raza Gilani said "relief items in sufficient quantity" have been provided, many people say they have received no assistance from the government, only from local families.

Particular scorn has been heaped on the deeply unpopular President Asif Ali Zardari for pressing ahead with visits to Paris and London at the height of the disaster.

Zardari was due to meet British Prime Minister David Cameron on Thursday for dinner ahead of formal talks on Friday, and the floods are expected to be on the agenda.

An international campaign is stepping up, and on Thursday the US pledged an extra 25 million dollars in flood aid, to add to the 10 million it had already promised.

The US also began flying military helicopter relief missions to the worst hit northwestern regions on Thursday, the US embassy said.