Hundreds of thousands of Pakistanis were fleeing a southern district Friday after the bloated Indus River crushed a levee and flooded new areas, officials said.

The United Nations said as many as one million people have been displaced in the south since midweek.

The latest evacuations came after the Taliban issued a veiled threat against foreign aid workers helping out after the floods, a development that stands to complicate one of the largest relief efforts the world as ever seen. More than eight million people are in need of emergency assistance across the country.

In the southern city of Thatta, around 175,000 people — around 70 per cent of the city's population — were believed to have packed up and left overnight, said Manzoor Sheikh, a senior government official. Authorities were trying to repair the broken levee and arranging transport for people trying to leave.

UN spokesman Maurizio Giuliano said according to reports received by the world body, around one million people were displaced in Thatta and Qambar-Shadadkot districts since Wednesday.

It is difficult to verify figures given by the authorities, however, partly because of poor or old census data and partly because of the difficulty of tracking migrations over such wide swaths of territory. Many of the areas are hard to reach because of the water, and people may have left their homes well before the evacuation orders.

UN aid agencies along with a host of other relief groups have been rushing people and supplies to affected regions as the flooding has lashed Pakistan over the past month.

Situation in Sindh province deteriorating

The situation in Sindh "is getting from bad to worse," Giuliano said. "We are delivering [aid] faster and faster, but the floods seemed determined to outrun our response."

Pakistanis line up at a food distribution point for families displaced by flooding at a temporary camp, in Sukkar, Sindh province, southern Pakistan on Thursday. (Kevin Frayer/Associated Press)

Also at risk in Sindh province are many historic graves, tombs and other sites linked to the Mughal Empire that once ruled the subcontinent.

The floods began with the onset of the monsoon and have ravaged a massive swath of Pakistan, from the mountainous north to its agricultural heartland. Almost 17.2 million people have been significantly affected by the floods and about 1.2 million homes have been destroyed or badly damaged, the UN has said.

The Pakistani Taliban on Thursday hinted that they might attack foreign aid workers, a swelling number of whom have been landing in the country to help with the crisis. The militant network has a history of attacking aid groups, including agencies under the UN umbrella.

Pakistani Taliban spokesman Azam Tariq claimed Thursday that the U.S. and other countries that have pledged support are not really focused on providing aid to flood victims but had other motives he did not specify.

"Behind the scenes they have certain intentions, but on the face they are talking of relief and help," Tariq told The Associated Press by telephone from an undisclosed location. "No relief is reaching the affected people, and when the victims are not receiving help, then this horde of foreigners is not acceptable to us at all."

Pakistani villagers flee their homes with their livestock after flooding in Thatta near Hyderabad, Pakistan, on Thursday. (Pervez Masih/Associated Press)

He strongly hinted that the militants could resort to violence.

UN humanitarian chief John Holmes said the UN remained committed to helping flood victims in Pakistan.

"We will obviously take these threats seriously as we did before, and take appropriate precautions, but we will not be deterred from doing what we believe we need to do, which is help the people of Pakistan," he told a news conference at UN headquarters in New York.

Other aid organizations noted that Pakistan has long been a high-risk environment for foreigners, and said their security plans took such concerns into account. Such groups rarely give specifics on their security procedures.