Authorities in flood-hit Pakistan strategically breached the country's largest freshwater lake on Sunday, a minister said, displacing up to 100,000 people from their homes but saving more densely populated areas from gathering flood water.

Record monsoon rains and melting glaciers in Pakistan's northern mountains have brought floods that have affected 33 million people and killed at least 1,290, including 453 children. The inundation, blamed on climate change, is still spreading.

Manchar Lake, which is used for water storage, had already reached dangerous levels, and the increased pressure posed a threat to surrounding areas in the country's southern Sindh province, Sindh Irrigation Minister Jam Khan Shoro said.

He said about 100,000 people would be affected by the breach in five councils, but it would help save more populated clusters and also help reduce water levels in other, harder-hit areas.

"By inflicting the breach we have tried to save Sehwan town. Water levels on Johi and Mehar towns in Dadu district would be reduced by this breach in the lake," Shoro told Reuters on Sunday.

It was not clear how many of the 100,000 asked to leave their homes would actually do so.

Aside from historic rainfall, southern Pakistan has had to contend with increased flooding as a surge of water flowed down the Indus river.

The country has already received nearly three times the 30-year average rainfall in the quarter through August, totalling 390.7 millimetres (15.38 inches). Sindh province, with a population of 50 million, was hardest hit, getting 464% more rain than the 30-year average.

Being downstream on the Indus river, the southern parts of the country have witnessed swelling river waters flowing from the north. Pakistan's limited dams and reservoirs are already overflowing and cannot be used to stop downstream flows.

Tarbela dam in the north-west, has been at capacity - 1,550 feet and 5.8 million acre feet - for weeks, according to NDMA data.

Downstream in Sindh, barrages are under pressure with the Indus river in high flood level, the NDMA said in its latest situation report.

(Writing by Gibran Peshimam; Editing by Jan Harvey)