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Fresh flooding in southern Pakistan has displaced almost a million people in the past 48 hours, the UN has said.

In Sindh province, 70% of the 300,000 residents of the town of Thatta have been forced to flee to safer areas after the Indus river burst its banks.

A UN spokeswoman said teams in the south were working around the clock.

Further north, floodwaters are starting to recede, revealing the full extent of the damage caused by the disaster that has affected some 17 million people.

A ´óÏó´«Ã½ correspondent joined a US military helicopter crew as they flew over a town in the Swat Valley and said roads and bridges had completely disappeared, leaving whole communities cut off.

Meanwhile, the UN said it was reviewing security for its staff after US officials warned that militants planned to attack foreign aid workers.

One Taliban spokesman told Associated Press news agency that the presence of foreign organisations was "unacceptable". However, there have been no attacks since the humanitarian crisis unfolded last month.

On Thursday night, the Pakistani authorities ordered people to leave several towns and cities in the south of the country as a flood surge breached more embankments on the Indus river.

Information Minister Qamar Zaman Kaira said low-lying areas of Sindh province would remain under water for the next two weeks or more. Out of its 23 districts, 19 have so far been badly affected by the floods.

"The embankment which was around the outskirts of Thatta has broken and it creates many difficulties," he told reporters in Islamabad on Friday.

The army and navy were both involved in the overnight evacuation of more than 200,000 people from the town, 70km (45 miles) east of Karachi.

Engineers are now trying to repair the 6m (20ft) breach before the centre of Thatta is swamped.

The surrounding towns of Sujawal, Mir Pur Batoro and Daro - with a combined population of 400,000 - had already been evacuated.

The ´óÏó´«Ã½'s Chris Morris witnessed a flood barrier breach near the city of Shahdadkot on Thursday. He said that as a breach widened, a series of fields rapidly filled up, taking on the appearance of an inland sea.

A spokesman for the UN Office for the Co-ordination of Humanitarian Affairs (OCHA) in Islamabad said nearly a million people had been displaced by flooding in Sindh alone in the past 48 hours.

"The magnitude of this crisis is reaching levels that are even beyond our initial fears," Maurizio Giuliano told reporters on Friday.

"The number of those affected and those in need of assistance from us are bound to keep rising."

"The floods seem determined to outrun our efforts. About one month from the onset of the floods, we don't know when we will see their end, as the disaster is still unfolding," he added.

The monsoon floods started in the mountainous north and have steadily surged south, destroying 1.2m homes and damaging 3.2m hectares (7.9m acres) of farmland - about 14% of Pakistan's land under cultivation.

The Pakistani government has so far confirmed that 1,600 people have been killed, but officials warn that millions are at risk from disease and shortages of food and water. The death toll may also rise as floodwaters recede.

The ´óÏó´«Ã½'s Jill McGivering, who visited Swat valley in northern Pakistan on Friday, says that as the water disappears, the full extent of the damage is becoming clear.

As she looked down on the town of Kalam, she saw the first signs of people starting to rebuild - a flimsy wooden bridge was erected over the Swat river, which wiped out everything in its path a few weeks ago.

All the bridges along the valley had disappeared, the marketplace was in ruins, and hundreds of hotels in the popular tourist town had been destroyed or partially-damaged, our correspondent added.

"There is also a problem of being connected to the outside world because, as well as losing the bridges, they also lost the roads and that means the only way of supplying the people of the town at the moment is by air," she said.

"I've just come in on a helicopter which brought in quite a lot of food - big sacks of flour, boxes of fortified biscuits and cooking oil - and about 50 men from this area came dashing forward to take what they could."

About $325m of the $459m sought by the UN in a flash appeal had been either contributed of pledged by foreign donors, while an additional $600m has been provided or promised outside the appeal, the UN's emergency relief co-ordinator, John Holmes said on Thursday.

"We're approaching $1 billion with funds offered or already contributed inside and outside the appeal for this crisis," he told reporters. "That's a reasonable response, but we certainly need more."

The International Monetary Fund, which has a $11bn loan programme with Pakistan, said the group was "looking at all options" to help.

If you would like to make a donation to help people affected by the floods in Pakistan, you can find information about how to do so by clicking here.