Flood survivors have a meal at a roadside in Thatta near Hyderabad, Pakistan, on Saturday. (Shakil Adil/Associated Press)

Floodwaters broke through the levees protecting a southern Pakistani city again on Saturday, prompting more than 175,000 people to leave their homes in search of higher ground.

The evacuation of roughly 70 per cent of Thatta's population began overnight after the latest levee breach, caused by the Indus river overflowing its banks in Sindh province.

The river is raging at 40 times its normal volume.

Many evacuees decided to camp out along the main road from Thatta while others kept moving in buses, cars, trucks and ox-drawn carts.

Taking shelter in graveyard

Thousands have headed for the high ground of an ancient graveyard for Muslim saints. The Makli Hill burial ground is not believed to be in danger of flooding.

The UN reports that around one million people have been displaced in Thatta and Qambar-Shadadkot districts since Wednesday because of floodwaters.

The floods began in the mountainous northwest about a month ago with the onset of monsoon rains and have moved slowly down the country toward the coast in the south, inundating vast swaths of prime agricultural land and damaging or destroying more than one million homes.

More than eight million people are in need of emergency assistance across the country.

U.S. officials announce Friday they would be sending 18 more helicopters to Pakistan by mid-September to help with flood relief efforts. These aircraft will supplement a fleet of 15 choppers and three transport planes already in use.

With files from The Associated Press