UN Secretary-General Ban Ki-moon has arrived in flood-ravaged Pakistan to boost relief efforts as the 20 million people made homeless grew increasingly desperate.
Survivors are fighting over food being handed out from a relief vehicle close to the town of Sindh, ripping at each other's clothes and causing such chaos that the distribution had to be abandoned.
"The impatience of the people has deprived us of the little food that had come," said Shaukat Ali, one of the flood victims waiting for food.
Waters 5ft deep washed through Derra Allah Yar, a city of 300,000 people on the border of Sindh and Baluchistan provinces, said government official Salim Khoso. About 200,000 had fled the city.
"We have to feed them, but we don't know how," he said.
Authorities said more flood surges were coursing down the River Indus and other waterways in southern Sindh province, inundating hundreds of other villages.
About 1,500 people have died in the disaster and more than 7.9 million acres (3.2 million hectares) of cotton, sugar cane and wheat crops destroyed. The International Monetary Fund has warned of dire economic consequences in a country already reliant on foreign aid to keep its economy afloat and one key to the war against al-Qaida and the Taliban.
The United Nations said the rate of diarrhoeal disease continued to increase among survivors. Cholera, which can spread rapidly after floods and other disasters, had also been detected in the north-west, where the floods first hit more than two weeks ago after exceptionally heavy monsoon rains.
"We are here like beggars," said Mukhtar Ali, a 45-year-old accountant living on the side of a highway along with thousands of other people. "The last food we received was a small packet of rice yesterday and 15 of us shared that."
The US has donated millions and sent military helicopters to rescue stranded people and drop off food and water. Washington hopes the assistance will support a pivotal regional ally and help improve its image in the country - however marginally - as it seeks its support in the battle against militancy.