Three flood victims float their belongings in Dera Allah Yar, Jafferabad District, Pakistan, on Monday. Reuters

As many as 33 million people have been affected, with at least 1,325 dead, including 466 children, in the floods brought by record monsoon rains and melting glaciers in Pakistan's northern mountains, national disaster officials have said.

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

"We fear the situation could deteriorate," said Indrika Ratwatte, the agency's director for Asia and the Pacific, adding that Pakistan's weather officials forecast more rains for the coming month.

A victim of unprecedented flooding from monsoon rains sits in front a shop in Jaffarabad, Pakistan, on Monday. AP

"This will increase challenges for flood survivors, and likely worsen conditions for nearly half a million displaced people, forcing more to abandon their homes."

A key concern is the Manchar freshwater lake in the southern province of Sindh, which is dangerously close to bursting its banks.

"We have widened the earlier breach at Manchar to reduce the rising water level," provincial irrigation minister Jam Khan Shoro told Reuters on Monday.

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.

The UNHCR is working with Pakistani authorities to step up humanitarian supplies if more people are displaced in the area, Ratwatte added, while the foreign ministry said three more UN relief flights arrived on Tuesday.

Internally displaced flood-affected people gather outside their tents at a makeshift camp in Sukkur, Sindh. AFP

"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.

Victims of unprecedented flooding struggle to transport their belongings in Jaffarabad, Pakistan, on Monday. AP

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.

Pakistani authorities restored power on Tuesday to towns and cities along the Afghan border, where hundreds of thousands of people have struggled without electricity for weeks.

Reuters