The World Food Programme has warned that flood-ravaged Pakistan faces a "triple threat" after the worst disaster in the country's history left eight million people dependent on aid to survive.

As floodwater in Pakistan displaces tens of thousands of people from their homes every day, officials warn that millions are at risk from food shortages and disease, adding to the humanitarian crisis (AFP) Torrential rains triggered massive floods that have moved steadily from north to south over the past month, engulfing a fifth of the country and affecting 17 million of Pakistan's 167 million people.

The floods have washed away huge swathes of the rich farmland on which the country's struggling economy depends.   

"There is a triple threat unfolding as this crisis widens and deepens," Josette Sheeran, the World Food Programme chief, said at a conference with other United Nations officials in the Pakistani capital, Islamabad, after visiting flooded areas on Wednesday.  

"People have lost seeds, crops and their incomes, leaving them vulnerable to hunger, homelessness and desperation - the situation is extremely critical," she said.

Anthony Lake, the head of Unicef, the UN children's fund, said that the disaster had affected nearly 8.6 million children.   

"In many ways, it is a children's emergency.   

"There is also a potential second wave of death from waterborne diseases.This is likely to get much worse if we can't reach people with clean water, adequate nutrition, sanitation and vaccination," he said.

Devastation 'staggering'

Al Jazeera's Imtiaz Tyab, reporting from the town of Mingora, said the waters were receding but the extent of the devastation is "staggering".

"It has been about a month since floods tore through Swat valley, the floodwaters have receded, but the devastation left behind is staggering. Areas are still not accessible by road, leaving far too many people isolated," our correspondent said.

Meanwhile floodwaters continued to sweep towards two small southern towns as authorities managed finally to plug a breach in defences across the Indus river in nearby Thatta city.   

Pakistani troops and city workers had been battling over the weekend to save Thatta, with most of the population of 300,000 fleeing the advancing waters.   

"Thatta city has been declared safe after a breach in the river caused by floods at nearby Faqir Jo Goth village was fully plugged," Hadi Bakhsh Kalhoro, a senior city official, told the AFP news agency.   

But he said the fast-moving waters that left the low-lying town of Sujawal submerged on Sunday were now threatening the towns of Jati and Choohar Jamali, where official warnings have been issued to  residents to evacuate.  

"We are making efforts to save the two towns which have a combined population of more than 100,000," Kalhoro said.

'Millions at risk'   
Most people had already returned to Thatta, Kalhoro said, on the western bank of the swollen Indus.   

But inundated Sujawal was mostly empty on Tuesday, as water flowed down its streets and troops offloaded rubber boats from their vehicles to rescue the remaining few, an AFP reporter on the scene  said.   

Jameel Soomro, a Sindh government spokesman, said that 147 people had been killed in the province, mostly as a result of disease triggered by the floods, and most of them women and children.  

Southern Sindh is the worst-affected province, with 19 of its 23 districts ravaged as floodwaters have swollen the raging Indus river to 40 times its usual volume.   

One million people have been displaced over the past few days alone.

Pakistan's government has confirmed 1,645 people dead and 2,479 injured but officials warn that millions are at risk from food shortages and disease.   

India on Tuesday offered another $20m in flood aid to Pakistan, boosting efforts to build goodwill between the estranged neighbours.