Torrential monsoon rains triggered massive floods affecting a fifth of the volatile country -- an area roughly the size of England -- where US officials have reported threats by the Taliban.

But UN humanitarian chief John Holmes pledged relief operations would continue.

"We will not be deterred from doing what we believe we need to do, which is to help the people of Pakistan," he told a news conference at the world body's headquarters in New York.

"Those threats existed before the floods and we've always known that the security issues are there," he said.

As the crisis deepened, villagers in the south fled from where the Indus delta merges with the Arabian Sea, trailing north in vans laden with furniture, crowded into buses, or in carts pulled by oxen. Some people were on foot, leading their livestock.

Water lined the road from Hyderabad to Thatta town, as workers frantically used bulldozers to dig embankments only just higher than the flooding, and where people camped out under open skies or in makeshift tents.

The catastrophe has already affected more than 17 million people and left eight million dependent on aid to survive.

The Pakistani government has confirmed that 1,600 people have been killed and 2,366 wounded, but officials warn that millions are at risk from diseases and food shortages.

In the southern province of Sindh, where the floods have washed away huge swathes of the rich farmland on which Pakistan's struggling economy depends, a senior administration official warned that fresh floods threaten the towns of Sujawal, Mirpur Bathoro and Daro.

Hadi Bakhsh Kalhoro told AFP that 400,000 had been warned of the danger posed by rising waters.

Administrator of Thatta district Manzoor Sheikh told AFP that 100,000 people were on the move to safer ground on Thursday.

"My estimate is that there were some 100,000 people moving just today towards safer places, and the movement of people was still continuing," Sheikh said.

The Sindh irrigation minister said waters were in danger of breaching a protective embankment in Garhi Khuda Bakhsh village, where former leaders Benazir Bhutto and her father, as well as her two younger brothers, are buried.

"We have strengthened the embankment because we don't want mausoleums of our martyrs to be flooded," the provincial minister, Saifullah Dharejo, told AFP.

On the Arabian Sea, authorities fear that coastal districts may flood in coming days, trapped by Indus river floods pushing south and rough seas.

"Most people have left Sujawal. I'm also leaving town with a heavy heart with my family," Mohammed Bakhshal, a 40-year-old farmer, told AFP.

"Local transporters are demanding a lot of money to carry our luggage and families to Thatta and Karachi. I had 15,000 rupees (175 dollars) in savings, which I'm now spending on transport."

The UN warned that 800,000 people in desperate need of aid had been cut off by the deluge across the country and appealed for more helicopters to deliver supplies to those people reachable only by air.

On the ground, the International Organization for Migration (IOM) says 4.5 million people remain in urgent need of shelter.

US State Department spokesman Philip Crowley told reporters in Washington, which has put Pakistan on the front line of efforts to beat back the Taliban in Afghanistan, of the threats facing those helping flood victims.

"We have information of the potential targeting of foreign relief workers in Pakistan, as well as government ministries," he said.

Crowley pointed specifically to the extremist group Tehreek-e-Taliban and voiced concerns that its militants may attack foreigners or government institutions involved in relief efforts.

Holmes said that the United Nations would take "appropriate precautions" against any threats.

The Pakistani Taliban have previously denounced all foreign aid for victims of the country's catastrophic flooding.

The International Monetary Fund (IMF) said Thursday it was exploring "all options" to help the country, which is seeking to ease the terms of a near 11-billion-dollar loan, including emergency funds.

Standard & Poor's said that Pakistan's sovereign credit rating would not be immediately impacted, expressing confidence the IMF would ease loan financing terms to the nation.

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