Authorities in Pakistan were battling on Monday to save a city in the flood-devastated southern province of Sindh after a mass evacuation as floodwaters threatened to wreak further havoc.

The near month-long floods have killed 1,500 people and affected up to 20 million nationwide in the country's worst natural disaster, with the threat of disease ever-present in the miserable camps sheltering penniless survivors.

Tens of thousands of people were evacuated from flood-threatened areas in the south on Sunday, including from Shahdadkot, with most of the city's 100,000 residents escorted to safety or making a getaway by whatever means possible.

"We are right now trying to protect Shahdadkot... 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".

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

Dharejo, however, stressed there was no threat to Hyderabad, the second-largest city in Sindh and Pakistan's sixth biggest overall with a population of 2.5 million.

Pakistan's weak civilian government has faced an outpouring of fury over sluggish relief efforts, while officials warn the country faces ruinous economic losses of up to 43 billion dollars.

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

Maurizio Giuliano, spokesman for the UN Office for the Coordination of Humanitarian Affairs in Islamabad, told AFP Monday that 1.5 million people were being treated for everything from respiratory and skin infections to diarrhoea.

The International Monetary fund is expected to begin talks with Pakistani officials on Monday on restructuring a 10-billion-dollar loan.

UN chief Ban Ki-moon on Friday 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 key ally in the fight against Islamic extremism, has given the most, followed by Saudi Arabia and Britain.

However, Louis-Georges Arsenault, head of emergency operations for UNICEF, the UN children's fund, said the international community could do far more.

"One of the major challenges we have, which is quite extraordinary, is the lack of level of support from the international community right now," Arsenault told the BBC.

"Our level of needs in terms of funding is huge compared to what we have been receiving even though this is the largest, by far, humanitarian crisis that we have seen in decades."

The United Nations has increased its initial estimate of the number of people without shelter from two million to six million.

In Shahdadkot, streets were deserted and all markets shut. A group of people was seen loading their belongings into a private vehicle before leaving, an AFP photographer in the city said.

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

Grocer Asghar Ali 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 said.

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

The IMF in 2008 approved a rescue package for Pakistan as the country struggled to cope with bloody attacks by Islamic radicals, 30-year-high inflation and fast-depleting reserves.

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.

Canada's government, which last week announced 32 million dollars for victims of Pakistan's floods, said Sunday it would give more aid by matching the amount donated by its citizens.