JAMSHORO: Pakistan was scrambling on Tuesday to widen a breach in its biggest lake and prevent the waters from overflowing to swamp nearby towns, so worsening unprecedented floods that have inundated a third of the South Asian nation.

Waters brought by record monsoon rains and melting glaciers in Pakistan's northern mountains have affected 33 million people and killed at least 1,325, including 466 children, national disaster officials have said.

"We have widened the earlier breach at Manchar to reduce the rising water level," Jam Khan Shoro, irrigation minister in the southern province of Sindh told Reuters late on Monday, referring to the lake, whose waters authorities seek to drain.

Already 100,000 people have been displaced from their homes in the effort to keep the lake from overflowing, an outcome that authorities fear could affect hundreds of thousands more.

"Till yesterday there was enormous pressure on the dikes of Johi and Mehar towns, but people are fighting it out by strengthening the dikes," district official Murtaza Shah said on Tuesday, adding that 80% to 90% of townspeople had already fled.

Those who remain are attempting to strengthen existing dikes with machinery provided by district officials.

The waters have turned the nearby town of Johi into a virtual island, as a dike built by locals holds back the water.

"After the breach at Manchar, the water has started to flow, earlier it was sort of stagnant," one resident, Akbar Lashari, said by telephone, following Sunday's initial breach of the freshwater lake.

The rising waters have also inundated the nearby Sehwan airport, civil aviation authorities said.

The floods have followed record-breaking summer heat, with the government and the United Nations both having blamed climate change for the extreme weather and the resulting devastation.