Pakistani authorities Sunday evacuated tens of thousands from flood-threatened areas in the south but insisted that the 2.5 million people of Hyderabad were safe from the nation's worst-ever inundation.

The weak civilian government has faced an outpouring of fury over sluggish relief efforts, while officials are warning the country faces ruinous economic losses of up to 43 billion dollars, ahead of IMF talks this week.

The month-long floods have killed 1,500 people and affected up to 20 million nationwide, according to official tallies, with the threat of disease ever-present in the miserable camps sheltering penniless survivors.

"We are right now trying to protect Shahdadkot town, which is threatened by the rising floodwaters," Sindh provincial irrigation minister Jam Saifullah Dharejo told AFP.

He said an embankment built to protect the city was under pressure from the waters and "we are trying to save the city from the unprecedented flood".

Dharejo said the Sindh government had already escorted most of Shahdadkot's 100,000 population to safety.

"But there are still some people stranded in these villages (around Shahdadkot) and we are making efforts to rescue them," he added.

Dharejo, however, stressed there was no threat to Hyderabad, the second-largest city in Sindh and Pakistan's sixth biggest overall.

"There is nothing of the sort... Hyderabad is so far safe despite the growing pressure from floodwaters. We have strengthened embankments around the city," the official said.

Streets were deserted and all markets shut in Shahdadkot town and a group of people were seen loading their belongings into a private vehicle before leaving their homes, an AFP photographer in the town said.

"People have migrated to safer places as they are afraid that the floodwater may inundate this town," a local farmer, Mehram Ali told AFP.

He said that majority of people have gone to Larkana or Sukkur towns, adding he had come back to Shahdadkot from Larkana to see if "my town is safe".

Asghar Ali, who owns a grocery store in Shahdadkot, was hurriedly packing up his luggage to leave the area.

"I cannot believe my eyes when I look at the empty town, which used to hum with activity just until a few days ago," he told AFP.

The International Monetary Fund said it would meet Pakistani officials in Washington this week to discuss the impact of the floods, which have devastated the country's southern agricultural breadbasket and its textiles industry.

Pakistan may reportedly ask the IMF to ease the terms of a 10-billion-dollar loan, which since 2008 has helped to prop up the enfeebled economy.

Millions of flood survivors in desperate need of food, shelter and clean drinking water meanwhile require humanitarian assistance to survive, as concerns grow over potential cholera, typhoid and hepatitis outbreaks.

UN chief Ban Ki-moon has praised the global community as emergency donations for Pakistan neared 500 million dollars, but warned the country faces "years of need".

The United States, which has made the nuclear-armed nation a cornerstone ally in the fight against Islamic extremism, has given the most, followed by Saudi Arabia and Britain.

On Friday Ban welcomed the donations, but warned: "We must keep it up. Pakistan is facing weeks, months and years of need."

"It is very likely that the need for donations will strongly increase because... the number of people in need of immediate humanitarian aid has risen from six to eight million," Maurizio Giuliano, spokesman for the UN Office for the Coordination of Humanitarian Affairs in Islamabad, told AFP.

The UN has increased its initial estimate of the number of people without shelter from two million to six million, he added.

"We have more than doubled the rate at which we are delivering relief but, since August 11, the number of people who need emergency shelter has undoubtedly more than tripled. We are in a race against time."

The UN World Food Programme said it urgently needed helicopters to get food to millions of flood victims who remain cut off by the high waters, although weather forecasters say the monsoon systems are easing off.

The WFP warned that the floods have killed or are threatening millions of livestock, and launched an urgent appeal for animal feed.