Pakistan scrambled on Tuesday to widen a breach in its biggest lake to prevent it from overflowing amid unprecedented floods that have inundated a third of the South Asian nation, as the United Nations warned of more misery to come.

Flooding, brought by record monsoon rainfall and glacier melt in the north, has impacted 33 million people and killed at least 1,325, including 466 children, the national disaster agency said.

About 636,940 displaced people have been housed in tent villages, it said, adding the raging waters had swept away 1.6 million houses, 5,735 kilometers of roads, railways, 246 bridges, telecommunication systems, 750,000 livestock, and inundated over 2 million acres of farmland, destroying both standing and stored crops.

Prime Minister Shehbaz Sharif's cabinet on Tuesday boosted funding for cash handouts for the 1.2 million most affected families to 70 billion Pakistani rupees ($315.32 million), his office said.

The agency told the cabinet that 81 districts out of a total of 160 had badly been affected by the floods.

Reuters' drone footage over Sindh province showed agricultural and residential areas completely submerged in water, with just the tops of trees and buildings visible.

Rice fields resembled massive lakes of several miles in diameter, aerial video footage by the Pakistani military showed.

Officials have estimated the economic cost of the losses at a minimum of $10 billion.

With more rain expected in the coming month, the situation could worsen, a top official of the United Nations' refugee agency (UNHCR) warned.

"This will increase challenges for flood survivors, and likely worsen conditions for nearly half a million displaced people, forcing more to abandon their homes," said Indrika Ratwatte, the agency's director for Asia and the Pacific.